Hydrogeological context

The deposits of Quaternary age blanket the alluvial plain as sand, gravel, and clay occurring mostly as alluvial deposits. These deposits make up the primary aquifer for the Tuoshigan alluvial plain and Kumalake proluvial fan. The thickness of the alluvium is general more than 250 m (Liang et al., 2003). The upper parts of the plain are generally composed of gravel and coarse sand, and the lower parts are typically made up of fine sand and clay. There is a phreatic water layer and two confined water layers at the lower portion of Kumalake proluvial fan within 200 m depth underground. The first confined layer is ten to fifteen meters underground, and the second confined layer is about fifty meters underground. The water table depth in the study area ranges from two to four meters (Tang et al., 2004). There are intense surface water and groundwater transformations inside the plain. The mountain torrents and springs usually percolate through the loose dry soil and transform into groundwater before they can reach the main stem. The confined water then spurts out as artesian in the plain. Part of the artesian spring water is diverted for irrigation by local population, and the remaining water flows to the main stem.

The mountain torrents and the springs are not the only sources of the artesian spring. Research shows that large volumes of river water penetrate into groundwater at the mountain gaps and becomes artesian around the conjunction of the two rivers (RDI, 1989). The use of artesian spring reduces the water diversion from the river main stem. Fig. 3 compares the river water diversion per unit irrigation area in the Tuoshigan-Kumalake River plain with the river water diversion per unit irrigation area in the Akesu River delta, a neighbouring area with similar climate, geographical characteristics and cropping system.