Over the past twenty years, irrigation development such as reclamation of waste land and the consequent large water withdraws have diminished the stream flow to lower reaches and damaged the surrounding ecological environment (Feng et al., 2001; Xu et al., 2004). The ecosystems of the low reaches have been substantially altered by human interventions. The quantity of water available for irrigation has seriously decreased and lower reach ecosystems have been seriously compromised.
The lower reaches of the Tarim River have dried up along with Taitema Lake, the termination of the river, causing deterioration of the vegetation, serious desertification, and the disappearance of many plant and animal species. A project, which started in April 2000, was established to pump water into the dried section of the Tarim River and regulate the flux of the tributaries. The Akesu River, the largest tributary to the Tarim River, will be highly impacted by this project. The Akesu River, the largest tributary to the Tarim River, is supposed in response to the drying up of the low reaches. Though the studies on the groundwater hydrology system of the upper reaches of the Tarim River are urged and become a hot topic, the knowledge concerning the groundwater system of the Akesu River is still limited. The Water and Salt Monitoring Project of Akesu Subproject (WSMPAS, 1999-2002) made the most important study in this area, and the available hydrologic data were surveyed and summarized. Hu et al. (2004) studied the water balance of the Akesu alluvial plain with attentions on soil water in farmland. However there are still no descriptions of how the surface water and groundwater interact and how the groundwater system and drainage supply to wild plants and wetland.
Groundwater interacts with surface water in nearly all landscapes, ranging from small streams, lakes, and wetlands in headwater areas to major river valleys and seacoasts. Recharge is an important factor in evaluating groundwater resources but is difficult to quantify. The present discussion is limited to recharge to the water table as opposed to inter aquifer recharge. Recharge can be diffuse or localized. Diffuse recharge refers to the widespread movement of water from land surface to the water table as a result of precipitation over large area infiltrating and percolating through the unsaturated zone. Localized recharge refers to the movement of water from surface water bodies to the groundwater system and is less uniform in space than diffuse recharge (Alley et al., 2002). In arid area, precipitation seldom recharges groundwater due to little rainfall. In consideration of the annual precipitation of 57 mm in the Akesu alluvial plain, the diffuse recharge from precipitation could be very little (Tang, 2003). Water bodies, such as river, lake, reservoir and irrigation system etc, compose the main localized recharge. Li et al. (1997) performed several irrigation experiments in the study area. They observed that groundwater was recharged in response to individual irrigation events and reported water table raise of 0.2-0.5 m for one flood irrigation event. Their results showed that the amplitude of water table change of one year could be more than 1.79 m in paddy field due to irrigation. Recharge from irrigation is obvious because of flood irrigation and shallow water table. Although water table would rise due to flood irrigation, they found water table would drop down during the non-irrigation period in the experiments. This indicates water lost by evapotranspiration could considerably exceed precipitation and irrigation, leading to a corresponding drop in the groundwater table.
It is generally assumed that groundwater recharge occurs in topographically higher areas and groundwater discharge in topographically lower areas. This is true primarily for regional flow systems, but the superposition of local flow systems makes the interactions between surface and groundwater more complex (Winter, 1999). Usually, topographically higher areas are cultivated as irrigation area. And topographically lower areas, where the surface drainage is diverted and subsurface drainage recharges, become wetland in the Akesu alluvial plain. This same situation also occurs in other arid area (Hothem et al., 1994; Lemly et al., 1993; Lemly, 1994; Ohlendorf et al., 1987). We therefore coarsely consider that farmland and canal system as groundwater recharge area. The recharge can be estimated from canal loss and field infiltration.
Discharge areas occur in that part of the drainage basin where, the net saturated flow of groundwater is directed upward towards the water table (Freeze et al., 1969). In discharge areas, the groundwater level is at or near the ground surface. With little precipitation, water loss by evapotranspiration from non-irrigation area should come from groundwater. Everywhere without surface water supply should be groundwater discharge areas. The discharge can be estimated from the land evapotranspiration, which closely depends on the land use type. The natural habitat is the place where a plant species or plant community grows and which provides the conditions in which the plant can live (Klijn et al., 1999). Plant species therefore are used widely as groundwater discharge indicators (Batelaan et al., 2003). There are very clear relation between vegetation and groundwater discharge in arid and semi-arid regions. Groundwater discharge therefore can be estimated according to land use type.
The objectives of the study are to set up a methodology to quantitatively investigate the groundwater system in the Akesu alluvial plain, to describe water interaction between surface water and groundwater, and groundwater interaction between irrigation area and non-irrigation area. After reviewing some key elements of hydrology in study area relevant to this study (Section 2), a water budget methodology and its implementation for the Akesu alluvial plain are presented (Section 3). The results are discussed in Section 4, and some conclusions are drawn in Section 5.