Comparing phreatic evaporation at zero water table depth with water surface evaporation

Salt-affected soils are mostly found in irrigated areas within arid and semi-arid regions where the groundwater table is shallow.Soils of this type have become an increasingly severe problem because they threaten both the environment and the sustainable development of irrigated agriculture.A tool to estimate phreatic evaporation is therefore urgently required to minimize the salinization potential of salt-affected areas.In this context,phreatic evaporation at zero water table depth(E0)is a key parameter for establishing a model for calculating phreatic evaporation.The aim of this study was to explore the law of phreatic evaporation and to develop structurally rational empirical models for calculating phreatic evaporation,based on E0data of six types of soil(i.e.,gravel,fine sand,sandy loam,light loam,medium loam,and heavy loam)observed using the non-weighing lysimeter and water surface evaporation(E601)data observed using a E601 evaporator of same evaporation area with a lysimeter-tube at the groundwater balance station of the Weigan River Management Office in Xinjiang Uygur Autonomous Region,China,during the non-freezing period(April to October)between 1990 and 1994.The relationship between E0and E601was analyzed,the relationship between the ratio of E0to E601and the mechanical compositions of different soils was presented,and the factors influencing E0were discussed.The results of this study reveal that E0is not equal to E601.In fact,only values of the former for fine sand are close to those of the latter.Data also show that E0values are related to soil texture as well as to potential atmospheric evaporation,the ratio of E0to E601and the silt-clay particle content(grain diameter less than 0.02 mm)is negatively exponentially correlated,and that soil thermal capacity plays a key role in phreatic evaporation at E0.The results of this analysis therefore imply that the treatment of zero phreatic depth is an essential requirement when constructing groundwater balance stations to study the law of phreatic evaporation.

Stable Isotopes and Chloride Applied as Soil Water Tracers for Phreatic Evaporation Experiment

A phreatic water evaporation experiment,without rainfall influence,was designed to study the mechanisms of soil water movement through groundwater recharge to the unsaturated zone. Soil moisture content,chloride concentration,and δD and δ~18 O values of soil water were measured. Results showthat with decreasing soil moisture content,the chloride concentration of leachate( ρ_f(Cl)) in the capillary water layer decreases,whereas the ρ_f(Cl) value of the hanging and film water layers above the capillary water layer increases. With the combined δD and δ~18 O values,the soil water in the hanging and film water layers is influenced by evaporation,although a dry sand layer of 39 cm exists above the wet sand layer. The highest evaporation rate and the largest salt accumulation occur at a depth of about 39 cm in columns d,e,and f(Six polyvinyl chloride columns were assigned as column a,b,c,d,e,and f). We deduce that soil water migrates in the form of liquid water above the capillary water layer. In the experiment,a part of phreatic water consumed is used for the movement of soil water,whereas the other part is lost to evaporation. Soil water could continue migrating upward with prolonged experiment duration.

Analysis of evaporation of high-salinity phreatic water at a burial depth of 0 m in an arid area

High-salinity phreatic water refers to which with total dissolved solids（TDS）>30 g/L. Previous studies have shown that high salinity phreatic water evaporation is different at different depths. High salinity phreatic water evaporation under 0 m depth is the basis of the high salinity phreatic water evaporation studies. In this study, evaporation of high-salinity phreatic water at a burial depth of 0 m in arid area was investigated. New insights were gained on evaporation mechanisms via experiments conducted on high-salinity phreatic water with TDS of 100 g/L at 0 m at the study site at Changji Groundwater Balance Experiment Site, Xinjiang Uygur Autonomous Region in China, where the lithology of the vadose（unsaturated zone） was silty clay. Comparison was made on the data of high-salinity phreatic water evaporation, water surface evaporation(EΦ20) and meteorological data obtained in two complete hydrological years from April 1, 2012 to March 31, 2014. The experiments demonstrated that when the lithology of the vadose zone is silty clay, the burial depth is 0 m and the TDS is 100 g/L, intra-annual variation of phreatic water evaporation is the opposite to the variation of atmospheric evaporation EΦ20 and air temperature. The salt crust formed by the evaporation of high-salinity phreatic water has a strong inhibitory effect on phreatic water evaporation. Large volumes of precipitation can reduce such an inhibitory effect. During freezing periods, surface snow cover can promote the evaporation of high-salinity phreatic water at 0 m; the thicker the snow cover, the more apparent this effect is.

Effect of climate change on the trends of evaporation of phreatic water from bare soil in Huaibei Plain, China

When the soil condition and depth to water table stay constant, climate condition will then be the only determinant of evaporation intensity of phreatic water from bare soil. Based on a series of long-term quality-controlled data collected at the Wudaogou Hydrological Experiment Station in the Huaibei Plain, Anhui, China, the variation trends of the evaporation rate of phreatic water from bare soil were studied through the Mann-Kendall trend test and the linear regression trend test, followed by the study on the responses of evaporation to climate change. Results indicated that in the Huaibei Plain during 1991-2008, evaporation of phreatic water from bare soil tended to increase at a rate of 5% on monthly scale in March, June and July while in other months the increase was minor. On the seasonal basis, the evaporation saw significant increase in spring and summer. In addition, annual evaporation tended to grow evidently over time. When air temperature rises by 1 °C, the annual evaporation rate increases by 7.24–14.21%, while when the vapor pressure deficit rises by 10%, it changes from-0.09 to 5.40%. The study also provides references for further understanding of the trends and responses of regional evapotranspiration to climate change.

Solution and its application of transient stream/groundwater model subjected to time-dependent vertical seepage

Based on the first linearized Boussincsq equation, the analytical solution of the transient groundwater model, which is used for describing phreatic flow in a semiinfinite aquifer bounded by a linear stream and subjected to time-dependent vertical seepage, is derived out by Laplace transform and the convolution integral. According to the mathematical characteristics of the solution, different methods for estimating aquifer parameters are constructed to satisfy different hydrological conditions. Then, tile equation for estimating water exchange between stream and aquifer is proposed, and a recursion equation or estimating the intensity of phreatic evaporation is also proposed. A phreatic aquifer stream system located in Huaibei Plain, Anhui Province, China, is taken as an example to demonstrate tile estimation process of the methods stated herein.