Decoupling And Change Mechanism Of Soil Water Balance In The Changwu Loess Tableland

Climate and land use change in the Loess Plateau have significantly altered the eco-hydrological processes.Therefore,it is urgent to clarify the evolution mechanism of hydrological processes in a changing environment.At present,the research on the changes in individual hydrological variables,such as soil water and surface runoff is relatively sufficient.However,the difficulties in estimating deep drainage and partitioning evapotranspiration under deep loess,resulted in the lack of decoupling and quantitative research on the components of the water cycle.Therefore,it is necessary to develop new methods to promote soil water balance decoupling,and further analyze the mechanism of climate and land use change influencing water balance,to promote the sustainable management of water resources.Apple trees have been widely planted in Weibei dryland,which significantly improves economic benefits and greatly reduces soil water content.However,its effect on other soil water balance components and its mechanism is still unclear.Therefore,farmland and apple orchards of different stand ages in Changwu tableland were selected as the research objects,and the research was carried out through paired-point sampling,combining isotope tracing and hydrological simulation technology.Firstly,the variation characteristics and influencing factors of soil water under different land uses were discussed,and the applicability of the HYDRUS-1D model to simulate the variation of soil water in deep profile was further evaluated.Secondly,the optimization parameters of the HYDRUS-1D model were further constrained after the combination of hydrogen and oxygen stability and radioactive isotopes to decouple long-term soil water balance.Finally,the mechanism of the soil water balance components changes is systematically analyzed,and the possible changes in soil water balance are predicted in combination with different land use types and climate change scenarios.The main results are as follows:1.The applicability of the HYDRUS-1D model was evaluated,and it was found that it could better simulate the variability of soil water in deep profiles under farmland and orchard.Based on the dynamic monitored soil water content and meteorological data from 2011 to2013,HYDRUS-1D was used to simulate the vertical distribution and temporal variation of soil moisture in a 10 m deep profile under farmland and orchard,and the applicability of the model was evaluated.During the calibration and validation periods,the determination coefficient of the model ranged from 0.65 to 0.85,the efficiency coefficient of the model ranged from 0.55 to 0.83,and the root mean square error of soil moisture ranged from 0.01 to0.02 cm³ cm^-3.The simulated soil water contents had similar vertical distribution and temporal variation as the observation.The calibrated model can be used to effectively study the effects of land use change on soil water,but it still needs to be optimized in terms of deep drainage estimation and evapotranspiration partition.2.The long-term average soil water balance be decoupled by combining stable hydrogen and oxygen and radioactive isotopes,providing a benchmark for the optimization of HYDRUS-1D decoupling soil water balance.Based on the sample matching method,the composition of stable hydrogen and oxygen and radioactive isotopes in deep soil water profiles was determined.The deep drainage was estimated by the tritium peak method,and the evaporation loss ratio was estimated by the isotope mass balance model,and then the evapotranspiration was partitioned.Compared with farmland,the soil water deficit under the orchard increased with tree ages,while deep drainage decreased significantly（p<0.01）.Evapotranspiration under farmland,17-year,and 26-year orchards were respectively 557.1,579.8,and 586.7 mm,accounting for 95.2%to 100.3%of the annual average precipitation,indicating that evapotranspiration increased and exceeded the annual average precipitation after farmland was converted to orchard.In addition,with the increase of stand ages,the proportion of soil evaporation to evapotranspiration decreased,while the proportion of vegetation transpiration to evapotranspiration increased,indicating that apple trees changed soil hydrological processes by increasing transpiration and reducing other hydrological fluxes.3.The HYDRUS-1D model is constrained based on the results of the isotope tracer,which improved the accuracy of evapotranspiration and soil water balance decoupling.The model parameters which are difficult to be obtained by experiment are optimized based on the results of the isotope tracer method,such as extinction coefficient,soil cover fraction,soil pressure water head,root water uptake response function,and the critical value of compensatory root water uptake.Compared with the results of the isotope tracer method,the errors of soil evaporation and vegetation transpiration were respectively 171±35%and 57±6%under the default parameters of HYDRUS-1D,while the errors were respectively 20±2%and 4±2%after optimizing parameters.The relative errors of the proportion of vegetation transpiration to evapotranspiration under the default and optimized were 58±5.8%and 6±0.8%,respectively.Therefore,with the optimized parameters model HYDRUS-1D,the effects of environmental change on soil water balance can be better evaluated.4.After simulating the proportion of vegetation transpiration to evapotranspiration at different timescales based on the optimized model,it was found that the dynamic changes of the proportion of vegetation transpiration to evapotranspiration in farmland and orchard and its relationship with environmental factors were timescale dependent.Using the HYDRUS-1D model constrained by isotope,the time series of the proportion of vegetation transpiration to evapotranspiration under farmland and orchard were generated and their control factors were analyzed.The annual average proportion of vegetation transpiration to evapotranspiration under farmland,17-year,and 26-year orchards from 1957 to 2017 were0.64±0.04,0.69±0.04,and 0.74±0.02,respectively.The proportion of vegetation transpiration to evapotranspiration under orchard was greater than farmland,but its variation was less than farmland.Precipitation dominated proportion variability of vegetation transpiration to evapotranspiration at the total wavelet domain（0～256 months）,annual（13～48 months）,and interannual（121～256 months）timescales,while vapor pressure deficit and relative humidity had dominant effects at monthly timescales（0～12 months）.Monthly variability in proportion of vegetation transpiration to evapotranspiration can be explained by relative humidity up to77%,while the interannual variability was controlled by environmental factors up to 99%（121～256 months）.These results help to clarify the response of evapotranspiration and its composition in ecosystems to changing environment.5.The soil water balance of the historical period from 1957 to 2017 was decoupled,and the dominant effects of climate and land use change were analyzed.With the increase in stand age,soil evaporation and deep drainage decreased significantly,while vegetation transpiration increased significantly.However,precipitation dominated the changes in soil water balance components.Each component of soil water balance had significant coherence with precipitation and mean temperature for consecutive 12 months,indicating that the correlation between climate and soil hydrology exists in seasonal variation.The PASC values of soil water storage changes,soil evaporation,and vegetation transpiration with precipitation are much greater than their values with average temperature.These results all emphasize the dominant role of precipitation under climate change in controlling soil water balance.6.The climate change scenario from 2011 to 2040 was constructed to simulate the change of soil water balance and analyze the dominance of influencing factors.It was found that climate change greatly changed evapotranspiration,while deep drainage was dominated by land use types.In the future,the climate of Changwu Loess Tableland will tend to be warmer and drier,and extreme climate events will increase.The average annual precipitation will not decrease significantly under the four emission scenarios,but the average annual maximum and minimum temperature will increase significantly by 1.6±0.4℃and 1.2±0.2℃,respectively.The warmer and drier climate will significantly increase evapotranspiration,and further lead to a significant decrease in deep drainage under farmland,while deep drainage under orchards is close to zero.The variance of precipitation and temperature has a greater influence on the soil water balance than the mean value,while climate change has a greater influence on the mean value of soil water balance components than the variance.In general,evapotranspiration is dominated by future climate change,deep drainage is controlled by land use types,and soil water storage changes respond in opposite directions to land use types and future climate change,indicating that adjusting land use patterns of partial deep-rooted to shallow-rooted plants can appropriately mitigate the adverse effects of climate change on soil water.In this study,the HYDRUS-1D was constrained and optimized based on the results of the isotope tracer,the model parameters which are difficult to be obtained by experiment are optimized based on the results of the isotope tracer method,such as extinction coefficient,soil cover fraction,soil pressure water head,root water uptake response function,and the critical value of compensatory root water uptake.Which further improved the simulation accuracy of the model,especially the accuracy of decoupling soil water balance and evapotranspiration.This can provide technical support for related research.Meanwhile,the quantified impacts of climate and land use change on soil water balance are helpful to strengthen the understanding of water cycle evolution under environmental change,and can provide references for land use planning and water resources management.