Estimating groundwater evapotranspiration rates using diurnal water-table fluctuations in a semi-arid riparian zone

In semi-arid climates, phreatophytes draw on shallow aquifers, and groundwater evapotranspiration (ET_G) is a principal component of groundwater budgets. Diurnal water table fluctuations, which often are a product of ET_G, were monitored in the riparian zone of Red Canyon Creek, Wyoming, USA. These fluctuations were higher in a riparian wetland (2–36 mm) than a grass-covered meadow (1–6 mm). The onset and cessation of water-table fluctuations correspond to daily temperatures relative to freezing. Spatial differences were due to vegetation type and specific yield, while temporal changes were due to vegetation dormancy. Ratios of ET_G to potential evapotranspiration (PET), K _c,GW, were similar to ratios of actual evapotranspiration (ET) to PET, K _c, in semi-arid rangelands. Before vegetation senescence, K _c,GW increased between precipitation events, suggesting phreatophytes pull more water from the saturated zone as soil moisture decreases. In contrast, K _c decreases with soil moisture following precipitation events as ET becomes increasingly water-limited. Error in ET_G is primarily from estimates of specific yield (S _y), which is difficult to quantify in heterogeneous sediments. ET_G values may be more reliable because the range of acceptable S _y is smaller than K _c and S _y does not change with vegetation type or soil moisture.     

Responses of phreatophyte transpiration to falling water table in hyper-arid and arid regions,Northwest China

Quantitative assessment of the impact of groundwater depletion on phreatophytes in (hyper-) arid regions is key to sustainable groundwater management. However, a parsimonious model for predicting the response of phreatophytes to a decrease of the water table is lacking. A variable saturated flow model, HYDRUS-1D, was used to numerically assess the influences of depth to the water table (DWT) and mean annual precipitation (MAP) on transpiration of groundwater-dependent vegetation in (hyper-) arid regions of northwest China. An exponential relationship is found for the normalized transpiration (a ratio of transpiration at a certain DWT to transpiration at 1 m depth, T_a*) with increasing DWT, while a positive linear relationship is identified between T_a* and annual precipitation. Sensitivity analysis shows that the model is insensitive to parameters, such as saturated soil hydraulic conductivity and water stress parameters, indicated by an insignificant variation (less than 20% in most cases) under ± 50% changes of these parameters. Based on these two relationships, a universal model has been developed to predict the response of phreatophyte transpiration to groundwater drawdown for (hyper-) arid regions using MAP only. The estimated T_a* from the model is reasonable by comparing with published measured values.© 2021 China Geology Editorial Office.