Use of Historic Persian Water System Data in Groundwater Models: Examples from Afghanistan and Emirates 1
2
Thomas J. Mack , Jack Eggleston U.S. Geological Survey,
[email protected], Pembroke, NH, USA 2 U.S. Geological Survey,
[email protected], Northborough, MA, USA 1
ABSTRACT Obtaining calibration data for models depicting conditions during pre-development periods can be challenging as such periods are characteristically data poor. This study presents two examples where simulation of historic water conveyance structures were used to help characterize historic, or pre-modern, conditions in calibration of groundwater flow models. Persian water conveyance structures, called ‘aflaj’ (or singular ‘falaj’) in the Emirates, ‘karezes’ in Afghanistan, or ‘qanats’ in some other regions, consists of hand dug tunnels, hundreds to thousands of meters long, that intersect upgradient water table surfaces and convey water downgradient for domestic use and irrigation of small farms. These structures can be identified, using remote imagery, by the presence of regularly spaced access holes that were used to create and maintain these tunnel systems. This type of water-supply system was commonly used throughout North Africa, the Middle East, and Asia for centuries. They are generally not used today having largely been supplanted by modern groundwater wells, and, in many areas these structures have failed because of water-table declines caused by groundwater development and/or climate change. Evidence of these systems from satellite imagery provides an indication of pre-modern water levels and this information was used in the calibration of two contrasting pre-development models in Afghanistan and the Emirates. INTRODUCTION Groundwater availability investigations often include a predevelopment simulation scenario to characterize groundwater conditions before modern pumping began or before the period of concern. Data used to parameterize and calibrate the pre-development model, such as withdrawals, stream baseflows and groundwater levels, are often sparse or unavailable, particularly in undeveloped parts of the world. Inclusion of withdrawals in a groundwater flow model provides valuable constraints for solution of otherwise non-unique groundwater flow problems. This study presents two examples where simulation of ancient water conveyance structures were used to help characterize historic, or pre-modern (pre mid-1960’s), conditions. Flows in the ancient water conveyance structures were used to help calibrate pre-development groundwater flow models in the Chakari Basin, Afghanistan and the Abu Dhabi Emirate, United Arab Emirates (UAE). Persian water conveyance structures, called ‘aflaj’ (or singular, ‘falaj’) in the UAE (fig. 1), ‘karezes’ in Afghanistan, or ‘qanats’ in some other regions, consist of a hand dug tunnel, hundreds to thousands of meters long, that intersects an upgradient water table Figure 1. Schematic cross section of a falaj (from surface and conveys water for use Tadros and Huneidi, 1994). downgradient. In this paper the term falaj (or plural aflaj) is used for the Abu Dhabi study area and karez for the Chakari Basin study area. The aflaj of the UAE are known to have been operational for thousands of years (Brook and Al Houqani, 2006; Tadros and Huneidi, 1994) while those in Afghanistan are less well known, with the oldest reported to be 300 years old (Rout, 2008), but are also likely to have been used for thousands of years in the region.
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Mack, T.J., and Eggleston, 2015, Use of Historic Persian Water System Data to Calibrate Groundwater Models: Examples from Afghanistan and Emirates, in Maxwell, R., Hill, M., Zheng, C., and Tonkin, M. Editors, MODLFOW and More 2015, Modeling a Complex World Proceedings: Golden, Colorado, USA, 2015, P. 403-406.
Incorporation of aflaj or karezs into the groundwater-flow models is being examined, both to simulate the primary water extraction method historically used in the study areas and as a means of improving calibration of pre-development groundwater-flow models. Adoption of modern pumping methods to extract groundwater and rapidly increasing populations and water use have changed groundwater conditions sufficiently such that most of the aflaj and karezs in both study areas are currently (2015) dry, inoperable, or abandoned (Brook and Al Houqani, 2006; Mack and others, 2014). Records show that although Abu Dhabi aflaj flows were stable through the 1960's, declines begin in the mid 1970’s coincident with increasing groundwater use, and by the early 1980’s many of the monitored aflaj had dried (Tadros and Huneidi, 1994). The flows in some Abu Dhabi aflaj are now augmented by pumped well water (Brook and Al Houqani, 2006). There is no information on karez flow in the Chakari Basin study area but, prior to 1980, groundwater supply wells were not common, and recently (2013), with several hundred wells in the basin, only 2 of 8 identified karezes were known to be functioning (Mack and others, 2014). Throughout Afghanistan it has been reported that more than 36 percent of karezes have dried and flows have declined at more than 83 percent of karezes (Shobair and Alim, 2004). The Afghan trends are due to a combination of factors in addition to increased water use and reliance on modern wells including a lack of maintenance due to decades of war, a multiyear drought in the early 2000’s, and possibly climate changes. INCLUSION OF AFLAJ AND KAREZES IN TWO GROUNDWATER-FLOW MODELS The aflaj and karezes in the two study areas have a typical structure consisting of a tunnel that conveys water to an oasis or farm and vertical access shafts (fig. 1) spaced 10 to 20 m apart in the Chakari Basin study area and 10 to 30 m apart in the Abu Dhabi study area. The most upgradient vertical access shaft is a source well termed a “mother well”. The falaj or karez skims water from the water table at its upgradent end. A typical falaj in the Abu Dhabi study area is 1 to several km long with the longest 10 km long. In the smaller Chakari Basin study area karezes were 100 to 1,250 m long. Mother wells were typically 10 to 20 m deep in both Abu Dhabi (Tadros and Huneidi, 1994) and southern Afghanistan (Macpherson, 2014). This is likely a function of the depth to the surface of the water table and the ability to dig wells by hand. In the study areas, aflaj and karezes were identified in satellite images from a series of regularly spaced access shafts, typically with a community or irrigated area at the down gradient end. Figure 2 shows a karez in Afghanistan indicated by the series of pits at the land surface. In both study areas, aflaj and karezes are dug into alluvial fan deposits adjacent to mountains or upland areas that discharge both groundwater and surface runoff to the adjacent alluvial fans. Simulation of aflaj and Karezes Groundwater flow models were developed for the 2 30,200 km Abu Dhabi study area (fig. 3), and the 2 391 km the Chakari Basin study area (fig. 4). The MODFLOW-2005 groundwater flow software (Harbaugh, 2005) was used with a grid cell size of 500 m in the Abu Dhabi model and 60 m in the Chakari Basin model. Both study areas are located Figure 2. Access shafts for a karez observed in alluvial sediments flanking mountain source rock. from the air in Afghanistan. The Abu Dhabi study area is located at sea level to about 400 m, with a mean annual precipitation of about 100 mm/yr, while the Chakari Basin study area is at an elevation of 2,050 to 3,300 m, with a mean annual precipitation of about 300 mm/yr.
Figure 3. Abu Dhabi, United Arab Emirates study area and simulated aflaj source wells Twenty aflaj were identified in the Abu Dhabi study area (fig. 3) and 8 karezes in the Chakari Basin study area (fig. 4). Mother well depth and pre-development (1964-1970) flow observations were available for 6 aflaj in the Abu Dhabi study area (Tadros and Huneidi, 1994). Other than visual observations, no information was available on the karezes in the Chakari Basin study area. The source well locations at both sites were identified from satellite photography and were assumed to coincide with the furthest upgradient land surface pit. Groundwater withdrawals by each falaj or karez were simulated by drain cells in the upper 15 m of each groundwater flow model. The drain elevations were set at the depth of the mother well, where known, or were assumed to be 10 m below the land surface. Mother well depths were likely to be only slightly below the pre-development water table. There is likely to be additional groundwater discharge downgradient of the mother well in the upgradient portion of the falaj or karez, and water leakage to the aquifer at lower portions of the falaj or karez, however the systems were simplified by simulation of only the mother wells. Water discharged by the falaj or karez was considered to be entirely consumed by evapotranspiration as water supplies are scarce in both study areas and were known to be very closely accounted for and efficiently consumed. Where information was available, the Abu Dhabi aflaj had measured flows of 11 to 57 l/s in 1964 (Tadros and Huneidi, 1994). The other simulated aflaj were assumed to be at the approximate median flow rate for aflaj in eastern Abu Dhabi (40 l/s). Simulated falaj and karez flows are being assessed against historic measurements and assumed flows with the weight of flow observations based on knowledge of the flow rate. Where no measurements are available flows and weights are based on the likely flow from remotely sensed indications of the size of the community supported. Different numerical representations of aflaj and karezes are being investigated, and the magnitude and importance of the groundwater withdrawals in water balance, are also being assessed.
SUMMARY Preliminary results show that inclusion of karez and falaj flows and structures, can improve the calibration of the predevelopment groundwater flow models being developed. In the Abu Dhabi United Arab Emirates study area, the large size of model grid cells, relative to the size of the aflaj systems, reduces the relative benefit of including falaj flow in the calibration. In the Chakari Basin, Afghanistan study area, the model grid cell size is small relative to karez size, so the flow representation is more accurate. The addition of such historic water use information can improve similar pre-development models in areas containing historic aflaj and karez water conveyance structures. ACKNOWLEDGMENTS The authors acknowledge the efforts of colleagues at the Environmental Agency of Abu Dhabi, Emirate of Abu Dhabi, United Arab Emirates and the Afghanistan Geological Survey, Ministry of Mines and Petroleum, Afghanistan, for knowledge and information they have shared with us. Figure 4. Chakari Basin, Afghanistan study area and simulated karez source wells REFERENCES Bohannon, R.G., 2010, Geologic and topographic maps of the Kabul south 30′ × 60′ quadrangle, Afghanistan: U.S. Geological Survey Scientific Investigations Map 3137, scale 1: 100,000, 2 sheets plus 34-p. pamphlet, http://pubs.usgs.gov/sim/3137/. Brook, M., and Al Houqani, H., 2006. Current status of aflaj in the Al Ain area, United Arab Emirates: Environment Agency Abu Dhabi, United Arab Emirates, 27 p. Harbaugh, A.W., 2005, MODFLOW-2005, The U.S. Geological Survey modular ground-water model—the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6-A16, variously p. Mack, T.J., Chornack, M.P., Flanagan, S.M., and Chalmers, A.T., 2014, Hydrogeology and water quality of the Chakari Basin, Afghanistan: U.S. Geological Survey Scientific Investigations Report 2014– 5113, 35 p., http://dx.doi.org/10.3133/sir20145113. Macphearson, G.L., Modeling ancient water supply systems: Groundwater flow to a kariz in Afghanistan. Unpublished report, University of Kansas, January 2015 Rout, B., 2008. How the Water Flows: A typology of irrigation systems in Afghanistan: Afghanistan Research and Evaluation Unit, Issue Paper Series, 80 p. Shobair, S.S., and Alim, A.K., 2004, The effects of calamities on water resources and consumption in Afghanistan: FAO Afghanistan, 11 p. Tadros, S.H., Huneidi, I., 1994. Aflaj of Al Ain area, Abu Dhabi Emirate: National Drilling Company Administrative Report 94-02, United Arab Emirates, 27 p.
