Effects of Groundwater Table Decline on Vegetation Transpiration in an Arid Mining Area: A Case Study of the Yushen Mining Area, Shaanxi Province, China

We studied the effects of coal mining on the shallow groundwater regime and vegetation in the Yushen mining area (YMA) in northwestern China using a Hydrus-1D numerical model, which was calibrated and validated with field observations, and then used to simulate the effects of groundwater table (GWT) changes on transpiration rates. The actual transpiration (T-a) was influenced by both GWT depth (GWTD) and precipitation. The ratios of T(a)and potential transpiration (T-p) vs. GWTD were fitted to a mono-exponential decay of GWTD with correlation coefficients as high as 0.98. Two threshold values, d(T-a/T-p)/d(H) <= - 2% and T-a/T-p = 0.30, were then used to calculate the unaffected and critical depths. The average unaffected and critical water depths were 3.60 and 2.20 m, respectively, which coincided fairly well with the field investigation. The sensitivity analysis showed that although reducing hydraulic conductivity (K-s) caused the T-a/T(p)to decline, decreasing the inverse of the air-entry pressure (alpha) caused the T-a/T(p)to increase, so variations in K(s)and alpha had only a minor effect. Considering that the pre-mining GWTD is between 0 and 4.0 m for about half of the YMA, the vegetation there might be degraded or lost by a GWT decline due to mining. Also, the influence of a GWT decline on transpiration was more obvious during a drought in the YMA. Therefore, effective measures must be taken to avoid a decrease in the GWT. For example, mine water should be reinjected and the mining height reduced to protect the YMA's shallow groundwater.