Contribution of Groundwater to Root Zone Soil Moisture in an Arid Woodland.

The Ejina basin is an extremely arid region in Northwest China that encompasses the lower reaches of the Heihe River, the second longest inland river in China.  Due to scarce rainfall, many plants in the Ejina basin depend on groundwater for sustenance.  However, groundwater resources have been declining due to groundwater overdraft and overuse of river water in the middle reaches of the Heihe.  The dominant natural tree species in the basin is P. euphratica  (diversifolious poplar).  In recent years, large areas of P. euphratica have withered, creating a visible symbol of ecological change and desertification in the Ejina basin. Ecological restoration requires restoring some of the flow in the Heihe and raising groundwater levels.  However, it is unclear what groundwater level is required to support P. euphratica.  In this work, we investigated factors affecting the contribution of groundwater to the P. euphratica root zone.  The HYDRUS-1D model was calibrated using data from an experimental P. euphratica woodland.  Model simulations were performed to determine the effect of groundwater depth on groundwater contribution to root zone soil moisture during the P. euphratica growth period, May 1 to October 31. The results indicate for a water table depth of 2.64 m (the actual depth in 2000), cumulative transpiration during the growing season is about .35 m, with 83 percent of that water originating from groundwater.  If the water table is dropped to 3 m, transpiration drops to .19 m and the groundwater contribution to 68 percent.  For a 2 m water table, transpiration is .48 m and the groundwater contribution is 87 percent.  These types of model calculations will aid the planning of restoration efforts in the Ejina basin.