Modification Of The White Method And Its Application To Groundwater Evapotranspiration

The climate in arid environments is characterized by low precipitation and highpotential evapotranspiration, and water requirement has increased dramatically innorthern China.Groundwater dependent ecosystems are widely distributed in aridand semi-arid areas. Evapotranspiration of groundwater (ETG) is often a significantcomponent of the water budget. ETGis notoriously difficult to measure directly dueto its inherent complexities and small scale spatial and temporal variations. Themain methods are water balance methods, extinction depth methods andgroundwater regime methods. For water balance methods, the precision depend onother water balance components. For small scale and short time, the calculationaccuracy is very low. For extinction depth methods，they use energy balance ormicrometeorological variables to determine potential evapotranspiration. Due todifferent hydrogeological conditions, the coefficient is difficult to acquire. Becauseof the advantages of diurnal water-table fluctuations, it is widely adopted by manyecohydrogeologists to estimate ETG. White (1932) published a method of estimatingETGon diurnal fluctuations in the groundwater level. The four key assumptions forthe White method are:(1) evapotranspiration by plants causes diurnal water-tablefluctuations,(2) evapotranspiration is negligible relative to the groundwater inflowbetween00:00and04:00,(3) the rate of groundwater inflow to the site is constantthroughout the day, and (4) specific yield is the volume of water extracted from thesaturated zone.The assumptions have been proven particularly problematic.Sometimes that may be introducing error in estimated results. To wide theapplication of White method, many research works have been done. Hays (2003)and Loheide (2005) published a method of estimating inflow. Loheide (2005)presented the readily available specific yield to assess White method.Based on the summaries of former researches on estimating ETGusing diurnalwater-table fluctuations, summarizing the precision and applications of three maingroundwater regime methods (the White method and the modified White method: the Hays method and the Loheide method). By modifying the Loheide method, anew method was developed to calculate evapotranspiration (ETG).(1) The quadraticfunction is found using the detrended observations.(2) The specific yield is givendifferent values between day and night.(3) As groundwater level contains muchnoise,a new technique was developed to removing these effects. Using ProcessingModflow to verify this modified method. The precision goes up by10%.The modified method was used to estimate groundwater evapotranspiration inthe Hailiutu watershed using the measured groundwater level. The result indicatesthat riparian zone evapotranspiration is17.2mm/d.The proposed method plays asignificant role in groundwater sustainable development in arid environmentstheoretically and practically.