Enrique R. Vivoni
Ralph M. Parsons Laboratory
Massachusetts Institute of Technology
[Abstract] [Introduction] [Recharge] [BARS] [MODFLOW] [Application] [Sensitivity Tests] [Results] [Conclusion] [References]
Enhanced aquifer recharge is one alternative to the water sustainability crisis occurring in many arid regions. Intermittent and intense rainfall events over an arid watershed can lead to short term surface water availability. Without the proper management of this water resource, the excess precipitation can be quickly lost to the high evaporative environment or lost from the watershed via runoff. By ensuring that the available surface water remains within the catchment in the form of stored groundwater, a sustainable flux of water is obtained for the region (Eltahir, 1996). Sustainability, defined in this perspective, allows a water resources manager to focus on ensuring a consistent yield corresponding to the climatically variable input.
This paper explores the possibility of enhancing natural aquifer recharge by implementing a variant to time tested hydrologic technologies used for centuries in various arid watersheds. The groundwater model, MODFLOW, is used to simulate an idealized catchment where a component of the Branched Aquifer Recharge System (BARS) have been suitably modeled. Comparisons to an identical catchment without the proposed system reveal the effect of the enhanced recharge to the aquifer levels. The efficiency of the aquifer recharge is measured as the sustainable supply of water at a supply well for various climatic conditions.
Modeling results suggest that BARS is superior to the homogeneous control case in distributing recharged water to the extraction sites. As the intermittency of the incident rainfall increases, so does the BARS performance during storm events, a feature that makes the branched recharge system an attractive alternative for arid and semiarid catchments. The simplified modeling of the recharge system discussed here is a first step towards combining our current hydrological understanding with hydrologic engineering technology to achieve a sustainable management of water resources in arid environments.
Keywords: Arid zone hydrology, aquifer recharge, water harvesting, sustainability, BARS, groundwater modeling, MODFLOW.
The hydrology of arid regions has not received as much attention as other climatic regions (Scanlon et al., 1997). Ironically, the focus of hydrologic research has not been the highly water stressed regions of the world where the importance of sustainable water resource management is greatest. It is only recently that the hydrologic community has attempted to understand the variability in fluxes and processes that occur within arid and semi-arid basins (Nash, 1999) through intensive field campaigns such as HAPEX-Sahel (e.g. Desconnets et al., 1997) and SALSA (e.g. Goodrich et al., 1998). In addition to smaller scale field experiments (e.g. Abu-Awwad and Shatanawi, 1997, Cattle, 1997), numerical modeling studies (e.g. Gore et al., 1998, Giao, 1999) and theoretical developments, more attention is recently being placed on the field of arid zone hydrology. A recent review by Scanlon et al. (1997) highlights some of the major issues in flow and transport within arid, unsaturated systems. A common thread in each new discovery has been the realization that hydrologic processes in an arid or semiarid watershed are distinctly different from their humid climate counterparts.
By definition, arid watersheds differ from humid watershed in their relative amount of precipitation as compared to the evaporative flux. Following the classification of Potter (1992), arid and semiarid regions are those subject to precipitation to evaporation ratios (P/E ratio) smaller than 0.5, and between 0.5 and 1.0, respectively. In addition to a lower net precipitation rate, rainfall events occur as infrequent, short duration, high intensity storms that bring a major portion of the annual rainfall to the surface during a very short period of time. Flash flood events may be a direct result of this type of storm over an arid or semiarid watershed. Under these conditions, the surface layer is unable to infiltrate the incident rainfall, resulting in precipitation excess surface flows that propagate rapidly through the watershed. Even for low intensity rainfall events, the surface crust that develops on arid watersheds can lead to significant surface runoff (Abu-Awwad and Shatanawi, 1997). Once on the surface, water in an arid region is subject to a high evaporative demand from the low humidity and high temperature environment. In many cases, the surface flows never reach the valley bottom (e.g. Lavee et al., 1997).
The thickness of the unsaturated zone is another distinguishing feature in arid watersheds (Scanlon et al., 1997). The presence of a deep water table allows for unsaturated water or vapor flux to occur in either vertical direction depending on the surface conditions. This may have the effect of preventing infiltrated water from percolating into the underlying aquifer. Coupled to the reduced availability of surface water, the various loss mechanisms in the unsaturated zone (evaporation from the surface layer, uptake by plant roots, upwards vertical vapor and water flux, etc.) reduce the possibility of water reaching the underlying deep aquifer. Thus, the natural aquifer recharge in an arid environment hinges on the downwards vertical flow of water in the unsaturated zone and an excess availability of water above the evapotranspiration demand.
Water availability in arid regions is both sporadic and highly variable in intensity. Consider a flash flood in desert setting. The input into the system is extremely small and infrequent, possibly a thunderstorm of high intensity lasting only a few hours, while its response, a flood wave propagating over a crusted surface, occurs rapidly and may be of great magnitude. In order to manage this water resource sustainably (i.e. maintain a constant flux of water into the system), one must exert some degree of control over the system by altering its response time and storage capacity. While the precipitation inputs are not alterable, many possibilities exist in modifying the watershed surface, aquifer properties and storage capacity. One alternative explored in this paper through a numerical modeling study is to enhance the natural recharge to the underlying aquifer.
Recharge Enhancements through Hydrologic Technology
Enhanced natural recharge is defined here as the flux of water into groundwater storage under conditions exceeding those imposed by the local hydrologic flux through the undisturbed unsaturated zone. In this regard, an enhancement to the natural recharge consists of providing mechanisms for an increased vertical flow across the unsaturated zone and into long-term aquifer storage. Increasing the recharge rate can be achieved either through a decrease in the water loss in the upper soil layers or an increase in vertical flux through the surface or within the unsaturated zone. The short and long term increase in aquifer storage can be provided either by a rising water table or engineered storage structures in the unsaturated zone. The enhanced aquifer recharge system proposed here is a low technology and low energy consumption alternative to the artificial recharge systems presently implemented in many arid regions of the world.
Artificial aquifer recharge has become a popular alternative to many water supply and wastewater disposal problems. Bouwer (1998) and Kimrey (1989) review the current state-of-the-art in engineered recharge systems in the United States, while Raju (1998) reviews the artificial recharge systems implemented in India. In addition, hydrologic technologies for water harvesting have been implemented in various forms in Europe, the Middle East, Northern Africa, and Eurasia. Water harvesting systems differ in the hydrologic approach taken to the issue of capturing and storing rain water. Some attempt to increase surface runoff into storage or collection areas by decreasing infiltration (e.g. van Wesemael et al., 1998, Ciuff, 1989), while others depend on increasing infiltration into the aquifer along the route of travel of surface runoff (Raju, 1998). Still others, rely on engineered structures to collect and pump water into the deep aquifer (e.g. Verma and Sarma, 1990). A review of the existing approaches reveals one major hydrologic deficiency, the exposure of the water to a high evaporative environment via surface runoff collection and ponding. The conceptual hydrologic plan proposed here relies on minimizing the exposure time to surface conditions of an incident rainfall by providing for an efficient mechanism of transporting precipitation into the deep aquifer.
Artificial recharge systems are not a new solution in arid regions. In fact, various hydrologic technologies were implemented hundreds of years ago in portions of Iran, Israel, Spain and India (see Fitzmaurice, 2000). Despite the lack of knowledge regarding hydrologic processes, earlier civilizations established in arid regions were able to modify the natural environment to provide a source of water for the population. Many of these systems evolved as a result of initial empirical observations and years of trial and error until an efficient engineering design was developed. Some of these systems are still in use today, while others are quickly being replaced by modern technology.
Very few attempts have been made to study these ancient systems using our present hydrological understanding of rainfall-runoff processes, infiltration, saturated and unsaturated flow and evapotranspiration. In one of the few studies of its kind, van Wesemael et al. (1998) describe the relationship between geomorphic location of a hydrologic engineering system and the water yield using a simplified curve number (Soil Conservation Service, 1986) approach to surface runoff. With the advanced knowledge concerning hydrological fluxes presently available, a critical look at these ancient hydrologic engineering systems should lead to: a) a theoretical understanding of the empirically arrived methods and b) improvements to these systems based on current hydrologic understanding.
Branched Aquifer Recharge System (BARS)
The proposed hydrologic engineering system is designed to enhance the natural recharge into a deep aquifer in an arid region by taking advantage of the time tested ideas implemented in ancient systems and incorporating modern-day hydrologic understanding. Hydrologic engineering systems can benefit greatly from the low-energy, sustainable systems developed in the past as a means of providing a long-term supply of high-quality water without the need for modern technology. The various elements of the proposed system are shown schematically in Figure 1 and consists of the following elements:
Hillslope collection
Orographic effects on precipitation tend to create different hydrometeorological conditions on hillslopes (Bras, 1990). The windward side of a mountain range may receive significantly more rainfall than the leeward side or regions at lower altitude. In an arid environment, this effect may be even more significant (van Wesemael et al., 1998). BARS takes advantage of the increased short-term availability of water on hilltops by placing hillslope collectors in high altitude locations within a watershed. These collectors are themselves placed in local depressions, assuring that surrounding flow converges into the sinkhole opening. The collector heads are designed to have a high infiltration capacity by placing a highly porous and conductive media. One potential material is a large-diameter rubble or gravel media. The gravel media also has a high heat absorbance which reduces the available surface energy for evaporation. Since the media is a poor substrate for vegetation growth, the possibility of transpiration losses are also reduced.
In order to maximize the infiltration into the hillslope collectors, these must be placed optimally to capture the maximum amount of hillslope runoff while reducing the exposure time for the surface flow. This may occur naturally along a surface-sealed hillslope, or may require surface treatment upstream of the hillslope collector. Regardless, the collector will effectively serve as a sink for runoff, increasing the infiltration capacity through the additional macropore space in the collector while assuring that ponding conditions on the surface do not occur. The macropore system enhances the vertical flux of water by providing for short term storage of the rainfall pulse and circumventing the tightly packed soil matrix. Time to ponding has been shown to be increased as a surface-sealed desert soil is perforated and its macroporosity increased (Cattle, 1999).
Branched network
With a substantial amount of surface flow reaching the hillslope collectors, the distribution lines must be sized to allow for a proper storage capacity without incurring in surface ponding. The distribution lines consist of an underground system of tunnels constructed between the hillslope and convergence collectors. The intended purpose is to provide regions of preferential flow and storage for runoff in the unsaturated zone. In an effort to use BARS as a low technology system, the distribution lines may be simple tunnels lined with an impermeable layer and filled with a permeable, highly conductive and porous media. One simple solution is a clay lined tunnel consisting of sand or gravel media. Similar systems called ghanats have been built in the arid regions of Iran to transport aquifer water from a hillslope to a population center by gravity flow (Farshad and Zinck, 1998). These systems consisted of vertical, open shafts connected by a underground tunnel along the hillslope gradient. Although effectively used for centuries, the incorporation of various elements in BARS should improve the efficiency of the underground tunnel system:
Topographic convergence
In a study of the relationship between topography and unsaturated flow in an arid region, Scanlon et al. (1999) showed that water fluxes were highest beneath topographic depressions that periodically flood as a response to local, intense, short duration storms. This is due to the fact that topographic convergence zones in a watershed provide a mechanism for surface and subsurface flow concentration. The relationship between surface topography and hydrologic flux in a watershed is a well understood and widely used concept in hydrological modeling. The TOPMODEL approach (Beven and Kirby, 1979) is a popular statistic-dynamic model for subsurface flow based on the distribution of topography in watershed. It predicts that flat depressions with large contributing areas will behave as convergence zones, in accord with the variable source area concept.
BARS takes advantage of the topographic gradient by placing larger collectors in topographic depression where subsurface flow from the distribution and storage lines is concentrated into one location with a minimal amount of energy expenditure. In addition, surface flow not captured by the hillslope collectors naturally flows into these convergence collectors whose surface infiltration conditions are designed similar to their hillslope counterparts. Flow concentration within the unsaturated zone assures a decrease in evaporation due to a lower surface area to volume ratio, the decrease in exposure to the high evaporative demand atmosphere and a reduced evaporation due to a lower temperature and increased moisture content.
The flow concentration within the unsaturated zone also assures that large rainfall pulses are stored underground, thus avoiding the evaporative demand of the surface. The entire branched network serves as a temporary storage of harvested water, while the convergence collector has the dual purpose of transmitting this water into the underlying aquifer via a deep percolation tunnel. It is in the convergence zone that the underlying aquifer will receive the collected runoff from the catchment. A groundwater mound will form underneath the convergence collector as the water is slowly transmitted through the phreatic aquifer. As the static water level in the aquifer increases, any overflow from the deep percolation tunnel may transported through a transmission line to lower parts of the watershed. This is illustrated in a schematic picture in Figure 2.
Transmission line and supply well
The BARS system is envisioned as a potential source of short-term and long-term water supply needs. This is achieved by using the collector components as long-term recharge units to the aquifer and short-term reservoirs for water. If the amount of stored water in the collector exceeds a particular water level, an overflow structure redirects the excess water into a transmission line that feeds a supply well. During period of water excess, the system can provide water quickly without the need of an energy consuming, deep extraction well. During period of water shortage, BARS is used exclusively to recharge the underlying aquifer, and extraction is performed using a conventional well system. Overflow structures regulate the short or long-term water storage and provide a sustainable water supply at various time scales.
A preliminary evaluation of the Branched Aquifer Recharge System is implemented in a numerical study using a groundwater model for a simple system. The major design criteria are incorporated into the groundwater model and a sensitivity test is performed to assess the effectivity of BARS in recharging an idealized arid zone aquifer. A more complete design of the BARS system would include a hydrologic analysis to determine the probability of rainfall occurrence and quantity in a region, modeling the rainfall-runoff response given the appropriate characterization of the surface conditions, a hydraulic design to determine the physical dimensions of the system components and the behavior of the system to different forcings, and modeling the movement of water within the BARS system and the effect on the aquifer level. In this preliminary study, however, the implementation of BARS within MODFLOW is limited to a simple system consisting of one hillslope collector with a branching network of storage and distribution lines into an underlying aquifer, as illustrated in Figures 5 and 6.
The MODFLOW groundwater flow model is applied in this study to an idealized arid watershed in order to evaluate the impact of the BARS hydrologic engineering system on the aquifer recharge. MODFLOW (USGS Modular Three-Dimensional Ground-Water Flow Model) is a recognized standard groundwater model that has been in development and use for the past three decades. It is capable of simulating a wide variety of groundwater flow and transport problems and its modular design has permitted developers to add capabilities as the need has arisen. For example, MODFLOW has been modified for aquifer remediation and biological transport, uses that were not envisioned during its initial conception. MODFLOW has been subject to rigorous peer review and scrutiny in courts, regulatory agencies, universities and within the consultant community, making it the most well known and tested groundwater model, and a suitable benchmark for this study.
For this study, a user interface program, Visual MODFLOW (Waterloo Hydrogeologic, Inc.) has been used to simplify the creation of input files, model operations and results. While Visual MODFLOW is a useful utility for setting up and running the model, it has very limited capabilities for presenting results. MODFLOW output was linked in this study to a Geographic Information System (ArcView GIS) for storage, manipulation and presentation of the spatially variable input and output data. The enhancement of MODFLOW through a GIS interface has recently been developed at the USGS (1998).
The preparation of the MODFLOW groundwater model consists of the following basic tasks:
Groundwater modeling studies of artificial recharge to aquifers in arid catchments have been performed by several researchers. The studies of Heller et al. (1999), Tompson et al. (1999), Bekesi and McConchie (1999), Munevar and Marino (1999) are some good examples of the various techniques and models used for this purpose. Modeling of the nature attempted in this study has not been performed specifically for the purpose of assessing recharge to an underlying aquifer. In particular, the implementation of the Branched Aquifer Recharge System in MODFLOW is based on the idea that the tunnel network can be modelled as zones of high hydraulic conductivity contained within layers or walls of low permeability. Since this is rarely seen in nature and has not been considered as an alternative for artificial recharge, modeling studies have not been performed. In this sense, this study is a first of its kind.
One potentially analogous system to BARS are the underground karstic caverns common to many regions of the world. Given their hydrological importance in karstic areas, modeling efforts of the large subterraneous caves and rivers have been made. A literature review on the subject has revealed that the scientific community is just beginning to understand the interaction of a groundwater system with these large, open, highly permeable cavities. Halihan and Wick (1998) present a simple reservoir model for the flow within the karstic aquifer based on some earlier field work by Halihan et al. (1998). A careful assessment of the similarities between both flow systems may lead to gaining some insight regarding the hydrologic behavior of BARS. Nevertheless, the use of a system such as BARS as an alternative for recharging the aquifer is a novel approach.
The implementation of BARS for enhanced natural recharge of an aquifer in MODFLOW consisted of creating a simple, idealized landscape and modeling the impact of the recharge system on the aquifer levels through various case studies and a control case. The idealized system is a three dimensional grid containing 50 cells by 50 cells in the horizontal plane and 10 layers in the vertical direction. The grid cell size is a constant spacing of 50 meters such that the simulation area is 2.5 kilometer by 2.5 kilometer in extent. The layers vary in depth from 1 meter spacing near the low-lying regions to tens of meters in the underlying aquifer. An idealized topography was created for the small arid watershed to simulate the effect of surface slope on the groundwater flow based on a simulated digital elevation model (DEM). The surface topography was implemented by specifying the elevation of a group of selected points and interpolating the grid cell elevations using a weighted inverse distance scheme to the nearest ten neighbors. The elevations in the DEM range from 50 meters in the corners of the idealized hillslope to 250 meters in the central portion. Using a hillslope length scale of approximately 1800 meters, the DEM has a nominal surface slope of 0.1 or 10%. A contour map of the topography is shown in Figure 3 with the location of the specified elevation points denoted by crosses. For a more realistic view of the surface topography, a three dimensional rendering of the idealized catchment is shown in Figure 4.
The hydrologic engineering system was implemented by varying the hydraulic conductivity and storage capacity within the various soil layers in the catchment to accommodate the design of a simple branched distribution recharge network, as shown in Figures 5 and 6. As can be seen from the top and side views of the BARS model, this preliminary study is considering an extremely idealized case. In an attempt to establish a benchmark for future work, the BARS system consists of one collector on a hillslope leading to four branched storage and distribution lines. This is thus a model of one possible BARS node within one hillslope receiving rain at a higher rate than its surroundings, either due to an orographic rain effect or to topographical convergence. An attempt was not made to model the interconnection of multiple nodes, the surface water concentration into a hillslope collector or an overflow structure or transmission line leading to a supply well.
Two cases were initially considered, a control case with homogeneous soil conditions over the arid watershed and a BARS base case with the implemented changes in soil parameters designed to model the recharge system. To properly set up the control case and the BARS base case in MODFLOW, the following parameters were specified based on literature values for various soil textures and climatic forcings to an arid zone watershed (Freeze and Cherry, 1979, Eagleson, 1970):
Isotropic hydraulic conductivity values (Kx = Ky = Kz = K):
The MODFLOW groundwater model was utilized to evaluate the effect of the Branched Aquifer Recharge System on aquifer levels during different climatic forcings. In order to assess how BARS impacts aquifer levels, a base case comparison was made between a homogeneous soil case and the BARS base case. The homogeneous case simulates the centralized or unbranched distribution of recharge to an aquifer while the BARS case simulates the effect of distributing the recharge over a larger region of the aquifer. The effect of the branching network is explicitly tested by comparing these two cases for a variety of climatic forcings. The input parameters for the comparisons are those outlined in the Model Application section. The models were run for the length of the simulation time and the results evaluated based on four criteria:
The scientific questions that this modeling study attempts to answer can be expressed as follows:
To provide some insight into these two questions, four climatic scenarios are considered for the BARS and homogeneous cases. A sensitivity study was performed to the precipitation forcing by altering the recharge time series. Four cases were considered:How does the distributed aquifer recharge provided by BARS impact aquifer levels under a variety of climatic forcings? What effect does rainfall intermittency have on its performance as compared to an undistributed recharge?
It is also interesting to evaluate how varying the spatial distribution of the aquifer recharge affects the available supply of water to the extraction wells. This evaluation is thus performed in tandem with the comparison of the two cases for the different climatic conditions. In this manner, the efficiency of BARS in increasing the available flux of water out of the aquifer, the sustainability condition defined previously, can be evaluated.
The efficiency of the aquifer recharge system can be measured by the
sustainable supply of water provided to a supply well. One way of evaluating
this effect in this modeling study is by comparing the water table levels
within various regions of the aquifer. The spatial and temporal variability
of the water table level can be analyzed through contour maps for a specified
time instance or by a time series plot of aquifer level at a specified
location. Alternatively, the amount of water transported between various
predefined zones can give an indication of the origin of the extracted
well water. This technique can supplement a mass balance calculation for
the closed system and is performed in MODFLOW through the ZONE
BUDGET package. Tables 1 and 2 presents the computed mass balance
for each climatic case and recharge system. In addition, the computed volumetric
flowrate between the three water budget zones (Catchment: Zone 1; Recharge
Zone: Zone 2; Supply Well: Zone 3) is shown.
|
(cubic meters) |
|
|
|
|
| Storage Change |
|
|
|
|
| Recharge |
|
|
|
|
| Well Extraction |
|
|
|
|
|
(cubic meters/day) |
|
|
|
|
| Zone 1 - 3 |
|
|
|
|
| Zone 2 - 1 |
|
|
|
|
|
(cubic meters) |
|
|
|
|
| Storage Change |
|
|
|
|
| Recharge |
|
|
|
|
| Well Extraction |
|
|
|
|
|
(cubic meters/day) |
|
|
|
|
| Zone 1 - 3 |
|
|
|
|
| Zone 2 - 1 |
|
|
|
|
The MODFLOW model runs for the climatic forcing cases in each recharge system are summarized here by using the techniques discussed in the Sensitivity Tests section. Figure 8 and Figure 10 show plan views of the water table depth contours for the Homogeneous (unbranched) and BARS (branched) recharge systems. The water table level is measured from a datum of z = 0 at the lowest soil layer. Unfortunately, the transfer of the VISUAL MODFLOW AutoCAD graphics to ArcView GIS results in the loss of the contour labels. For this reason, the purpose of these graphics is to qualitatively show the impact of rainfall variability on the simulated water table level for the two systems. More quantitative measures will be provided for specific observation wells within the catchment domain. Figure 9 and Figure 11 show side views of the water table depth and groundwater flow direction for a specific cross section through the BARS Recharge Head and the BARS Collector Head. These simulation results only represent the system behavior at a specific time, t = 180 days, the end of the simulation period. Changes in the groundwater table elevations, flow directions and magnitudes occur throughout the transient simulation. A more thorough analysis would be required to determine if case comparisons at other time periods lead to extremely different behaviors. The aquifer level time series at the three specific points suggest that this is not the case.
The aquifer level was monitored for all time periods at three locations: one supply well, one recharge head and the hilltop. Due to the radial symmetry inherent in the system, these results are representative of conditions elsewhere. The time series of aquifer level at these three locations are shown in Figure 12 for the Supply Well, Figure 13 for the Recharge Well Head and Figure 14 for the Hilltop. Each figure shows the results from the BARS and Homogeneous cases for the various climatic conditions discussed in the Sensitivity Test section. As previously discussed, the water level values represent the depth from the datum z = 0 at the bottom layer. In inspecting these figures, it is important to remember that an initial condition of Hi = 45 meters was uniformly applied to the catchment. In addition, the constant pumping rate in the four supply wells imposes a general decrease in aquifer levels throughout the catchment. This effect is only counteracted by the recharge supplied by the two systems. Is the distributed recharge provided by BARS more effective than the concentrated recharge occurring for the homogeneous case? Figure 15 and the results in Tables 1 and 2 should help in answering the first of our scientific questions.
Distributed Recharge Effect
A prominent effect of recharge distribution is observed due to the branching network of the Branched Aquifer Recharge System. The branching network transports a concentrated rainfall input occurring at a hilltop due to an orographic effect or in a depression due to topographic convergence to distant locations within the catchment. This recharge dispersal effect increases the aquifer level near the supply wells and enhances the amount of available of water for extraction. This can be clearly seen in Figure 15 where a comparison between the concentrated and the distributed recharge systems is made. The important conclusion provided by the head ratio is that the branching network increases the aquifer levels slightly for all times at the supply well, where it is most desired, at the expense of decreasing the levels more significantly in the recharge zone, where it is constantly being replenished. The calculated transport between the recharge zones and the aquifer as shown above in Tables 1 and 2 further support this conclusion. The volumetric flowrate from the recharge zone to the aquifer is consistently higher for the BARS case (Zone 2 - 1) for identical mass balances. This implies that BARS is more efficient in supplying recharge water to the extraction wells. By doing so, BARS increases the sustainable water supply available from the catchment, as implied by the increased groundwater mound areal coverage shown in the contour maps in Figures 8 and 10. Not only is the spatial extent of the groundwater mound increased due to the distributed recharge, but the temporal distribution of the peak head in the recharge well varies. This is most clearly seen for Case 1 in Figure 13 for the BARS case and Figure 14 for the Homogeneous case. The peak water table level occurs earlier in the simulation period at the recharge zone for the BARS case, suggesting that the system is capable of fulfilling short term water needs more quickly due to its distributed nature. Indirectly, the spatial and temporal variations suggests that the distribution of recharge over a branched network can lead to higher sustainable flux into the aquifer, even for the adverse conditions considered here. It must be noted that a major assumption has been made in that the homogeneous case is able to transport the same amount of water through the unsaturated zone as the engineered hydrological system. Due to the elements of the BARS design, this is highly unlikely. Finding that BARS is more efficient in transporting water to a supply well, even under the assumption of equal performance, is significant.
Climatic Forcing Effect
The temporal distribution of the incident rainfall has a profound effect on the spatial and temporal distribution of aquifer levels within the idealized arid catchment for both recharge systems. The first order effect is that an increase in the rainfall intermittency, keeping a constant rainfall volume and pumping rate, reduces the aquifer level at the supply well, as clearly demonstrated in Figure 12. Given that Cases 2, 3 and 4 only differ in the degree of intermittency, the clear decrease in aquifer levels in the supply well indicate the importance of the ratio of interstorm period length to storm period length. For a constant pumping rate, an increase in the rainfall intensity does compensate for a proportional reduction in storm duration. This is a significant finding that suggests that an arid watershed managed for water supply is more sensitive to the arrival frequency of storms than to the amount of rainfall incident upon its recharge zones. A second order effect of the rainfall intermittency can be observed in relation to Figure 15, the comparison of the system recharge performances. As compared to the constant recharge rate, intermittency reduces the effectiveness of BARS in providing an increased aquifer level to the supply well at the expense of maintaining higher levels in the recharge zone. As intermittency is increased among Cases 2, 3 and 4, however, the BARS efficiency increases during pulsed recharge periods of higher intensity. This is also evidenced by the increase in the volumetric flowrate from the recharge zone to the aquifer in Tables 1 and 2. The first order and second order effects mentioned above work in opposite directions. The overall decrease in aquifer levels at the supply well is due to the first order effect of having longer interstorm periods with a constant pumping rate. The magnitude of this effects hides the true sensitivity of the BARS system to rainfall intermittency, that expressed by the second order effect. As a rainfall pulse becomes more intense and less frequent, the recharge zones are more efficient in transmitting the pulsed recharge. Given the nature of storm arrival in arid watersheds, we expect that BARS would outperform the homogeneous case during the recharge events. Thus, discovering that the performance of BARS as a distributed recharge system is highly dependent on the temporal distribution of the incident rainfall is a significant finding in this study.
Currently, the use of artificial recharge to an aquifer is seen as a potential solution to the sustainable water supply needs of various arid and semiarid regions. One needs only to inspect the rising number of projects being designed and constructed in the semiarid southwest of the United States to appreciate the growing popularity of engineered recharge systems (ENR, 1999a,b; WATER/Engineering and Management, 1995, 1999). Other types of aquifer recharge and water harvesting systems have been implemented in many arid region over the past hundreds of years. Despite this wealth of engineering knowledge, a quantitative hydrological analysis of an artificial recharge system that considers the impact of the competing hydrological processes in arid regions has not been performed. This preliminary study takes a small step in that direction by presenting the conceptual design and initial modeling results of the Branched Aquifer Recharge System (BARS).
The Branched Aquifer Recharge System takes advantage of the hydrologic processes that favor the increase of vertical transport of water through the unsaturated zone, the concentration of flow by topographical constraints and the reduction of evaporative loss due to the high atmospheric evaporative demand. Four basic elements comprise the hydrologic engineering technology: hillslope collectors, a branched network of underground tunnels for storage and distribution, convergence zone collectors with an overflow structure to a transmission line and supply well. BARS is designed to combine the time-tested elements of ancient water harvesting systems (i.e. ghanats) with advances achievable due to our current understanding of hydrologic processes in arid catchments.
This modeling study concentrates on performing an initial investigation into the efficiency of the branched distribution system proposed for BARS on the aquifer levels near the zone of water extraction, as compared to a homogeneous, unbranched recharge to the same idealized arid catchment. The popular groundwater model, MODFLOW, is used for this purpose despite its limitations in modeling the surface hydrology and the unsaturated zone flow. By specifying the appropriate recharge conditions, the BARS system is mimicked within the MODFLOW modeling environment and tested under a variety of climatic forcings. The case studies demonstrated that the BARS system increases the amount of available water to the supply system and decreases the time for recharge water to be available. These two effects suggest that the BARS system is capable of increasing the sustainable flux of water into the aquifer even under identical system performances, an unrealistic expectation for the homogeneous, unbranched system. The case studies also demonstrated that the intermittency of the recharge into the system is a crucial parameter that governs the efficiency of BARS in transporting water to the demand sites. BARS is best suited to handle the highly intense and intermittent rainfall conditions expected in an arid watershed.
Acknowledgments
This study was performed to fulfill a term paper requirement for the class 1.714 Surface Hydrology offered at the Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, Ralph M. Parsons Laboratory by Prof. Elfatih Eltahir (Spring Term 2000). Fruitful discussions with E.A.B. Eltahir and D. Collins are acknowledged.
Abu-Awwad, A.M. and Shatanawi, M.R. 1997. Water harvesting and infiltration in arid areas affected by surface crust: examples from Jordan. Journal of Arid Environments. 37: 443-452.
Bekesi, G. and McConchie, J. 1999. Groundwater recharge modelling using the Monte Carlo technique, Manatawu region, New Zealand. Journal of Hydrology. 224: 137-148.
Bouwer, H. 1998. Issues in Artificial Recharge. Water Science and Technology. 33(10-11): 381-390.
Bras, R.L. 1990. Hydrology: An introduction to hydrologic science. Addison-Wellesly. Reading, Massachusetts.
Cattle. S.R. 1999. Efficacy of perforating the soil to capture and store rain during fallow in dry regions. European Journal of Soil Science. 50: 481-487.
Ciuff, C.B. 1989. Water harvesting systems in arid lands. Desalination. 72: 149-159.
Desconnets, J.C., Taupin, J.D., Lebel, T. and Leduc, C. 1997. Hydrology of the HAPEX-Sahel Central Super-Site: surface water drainage and aquifer recharge through the pool system. Journal of Hydrology. 188-189: 155-178.
Eltahir, E.A.B. 1996. Sustainable water resources: concept, definition and example. Ralph M. Parsons Laboratory Department of Civil and Environmental Engineering. Massachusetts Institute of Technology. Unpublished manuscript.
Engineering News Record. 1999a. Water Campus ready to begin Arizona Aquifer recharge. July. 14.
Engineering News Record. 1999b. California landowner proposes aquifer storage. September. 16-17.
Farshad, A. and Zinck, J.A. 1998. Traditional irrigation water harvesting and management in semiarid western Iran: A case study of the Hamadan Region. Water International. 23: 146-154.
Fitzmaurice, J. 2000. Ancient hydrologic technologies. MIT Class Project: 1.714 Surface Hydrology.
Freeze, R. A. and Cherry, J. A. 1979. Groundwater. Prentice-Hall. Englewood Cliffs, New Jersey. 604 pp.
Giao, P.H., Phien-Wej, N. and Honjo, Y. 1999. FEM quasi-3D modeling of responses to artificial recharge in the Bangkok multiaquifer system. Environmental Modelling and Software. 14. 141-151.
Goodrich, D.C., Chehbouni, A., Goff, B., MacNish, B., Maddock, T., Moran, S., Williams, D.G., Watts, C. 1998. An Overview of the 1997 Activities of the Semi-Arid Land-Atmosphere (SALSA) program. American Meteorological Society. Special Symposium on Hydrology. Phoenix, AZ.
Gore, K. P. , Pendke, M.S., Gurunadha, V.V.S., Gupta, C.P. 1998. Groundwater modeling to quantify the effect of water harvesting structures in Wagarwadi watershed, Parbhani district, Maharashtra, India. Hydrological Processes. 12. 1043-1052.
Halihan, T., Wicks, C.M. and Engeln, J.F. 1998. Physical responses of a karst drainage basin to flood pulses: example of the Devil's Icebox cave system (Missouri, USA). Journal of Hydrology. 204: 24-36.
Halihan, T. and Wicks, C.M. 1998. Modeling storm responses in conduit flow aquifers with reservoirs. Journal of Hydrology. 208: 82-91.
Hellers, J.A, Guertin, D.P., Miller, S.N. and Stone, J.J. 1999. GIS for watershed assessment: Integrated spatial and tabular data to derive parameters for a hydrologic simulation model (ARDBSN). Proceedings of 1999 ESRI International User Conference. July 26-30. San Diego, CA.
Lavee, H., Poesen, J. and Yair, A. 1997. Evidence of high efficiency water harvesting by ancient farmers in the Negev desert, Israel. Journal of Arid Environments. 35: 341-348.
Kimrey, J.O. 1989. Artificial recharge of groundwater and its role in water movement. Desalination. 72: 135-147.
Munevar, A. and Marino, M.A. 1999. Modeling analysis of groundwater recharge potential on alluvial fans using limited data. Ground Water. 37(5): 649-659.
Nash, D.J. 1999. Arid Geomorphology. Progress in Physical Geography. 23(3): 429-439.
Potter, L.D. 1992. Desert characteristics as related to waste disposal. In: Deserts as dumps? The disposal of hazardous materials in arid ecosystems. C.C. Reith and B.M. Thomson, ed. Univ. of N.M. Press. Albuquerque, N.M. 21-56.
Raju, K.C.B. 1998. Importance of recharging depleted aquifers: State of the art of artificial recharge in India. Journal Geological Society of India. 51. 429-454.
Rinaldo, A., Rodriguez-Iturbe, I. and Rigon, R. 1998. Channel networks. Annual Review of Earth Planetary Science. 26: 289-327.
Scanlon, B.R., Tyler, S.W. and Wierenga, P.J. 1997. Hydrologic issues in arid, unsaturated systems and implications for contaminant transport. Reviews of Geophysics. 35(4): 461-490.
Scanlon, B.R., Langford, R. P. and Goldsmith, R. S. 1999. Relationship between geomorphic setting and unsatured flow in an arid setting. Water Resources Research. 35(4): 983-999.
Soil Conservation Service. 1986. Urban hydrology for small watersheds. Technical Release 55, US Department of Agriculture, Washington, D.C.
Tompson, A.F.B., Carle, S.F., Rosenberg, N.D. and Maxwell, R.M. 1999. Analysis of groundwater migration from artificial recharge in a large urban aquifer: A simulation perspective. Water Resources Research. 35(10): 2981-2998.
van Wesemael, B., Poesen, J.,Sole Benet,A., Cara Barrionuevo, L. and Puigdefabregas, J. 1998. Collection and storage of runoff from hillslopes in a semi-arid environment: geomorphic and hydrologic aspects of the aljibe system in Almeria Province, Spain. Journal of Arid Environments. 40: 1-14.
Verma, H.N. and Sarma, P.B.S. 1990. Design of storage tanks for water harvesting in rainfed areas. Agricultural Water Management. 18: 195-207.
WATER/Engineering & Management. 1995. Aquifer recharge enhanced with rubber dam installations. January. 37-40.
WATER/Engineering & Management. 1999. Aquifer recharge: A natural
solution. January. 30-32.