Soil Water Movement And Distribution Under Typical Land Uses In Hilly Area Of Taihu Lake Basin

Taihu Lake Basin is presently confronted with accelerated water eutrophication,the run off of non-point nutrients is served as the main reason for the water eutrophication and the frequently outbreaks of blue algae in Taihu Lake Basin.Previous studies widely indicated that the migration of non-point nutrients is closely related to soil water movement and distribution.Thus,disclosing the process of soil water movement and distribution has great significance on revealing the mechanism of nutrient cycles and controlling non-point pollutants.In Taihu Lake Basin,the hill area is 6151.23 km2,and occupies 16.67%of the whole area.With the development of natural resource's exploitation,ecological land use has been drastically transformed into an economic one.Accordingly,the spatial and temporal distribution of soil moisture and subsurface flow has been changed apparently,causing great impacts on migration of nutrients.Recently,the eco-environmental problems have received considerable attentions in plain area of Taihu Lake Basin,while are less considered in hilly area of Taihu Lake Basin.Therefore,this dissertation focuses on the influence mechanism of water movement and distribution in soils when transformed from ecological land use to economic land use,hoping can provide theoretical basis on the land use optimization design of reducing and intercepting the run-off of non-point nutrients.Two adjacent typical land use patterns on a hillslope was selected in hilly area of Taihu Lake Basin in this study,one is tea garden(TG,Camellia sinensis(L.)O.Kuntze),a representative economic land use in Taihu Lake Basin,which is developed by bamboo forest land in 15 years ago;another is bamboo forest(BF,Phyllostachysedulis(Carr.)H.de Lehaie),a representative ecological land use.Firstly,the high spatial resolution soil moisture monitoring system on both these two land types(39 sampling sites on TG hillslope and 38 sampling sites on BF hillslope)was established by prortable timedomain reflectometry TRIME-PICO-IPH soil water probe;high temporal soil moisture data on limited sites(3 monitoring sites on TG and BF hillslopes,respectively)were obtained by soil water sensors EC-5(Decagon Devices Inc.,Pullman WA,USA).Secondly,by analyzing the influences of different dynamic factors(e.g.,precipitation,temperature,radiation)and static factors(e.g.,terrain indices and soil properties),the dominated controlling factors of the spatio-temporal distribution of soil moisture were identified.After that,applying Richards's equation,the high temporal resolution soil water movement model were established on both TG and BF hillslope.Based on temporal stability(TS)analysis and support vector machines(SVM),and integrating with manually measured soil moisture data and automatic monitoring moisture data,the high spatio-temporal resolution soil moisture distribution model.Then,based on the above analysis results,the subsurface flow fluxes on TG and BF hillslopes were estimated under multi-integrated scenarios of precipitation,soil texture and slope.Besides,the active regions of the variations of soil moisture distribution in different time scales(e.g.,year,season and month)were identified on these two land types,and the hot moment and hot spot of soil water movement on TG and BF hillslopes were discerned.Thereby,the influence of the change from ecological land(BF)to economic land(TG)on the process,mechanism and controlling factor of soil hydrology.Several results are shown as follows:(1)Remarkable differences of the spatio-temporal distribution patterns of soil moisture and the controlling factors were discovered between the TG and BF hillslopes.The spatial mean soil moisture is larger on BF hillslope(ranging from 0.103 cm3cm-3 to 0.296 cm3cm-3)than on TG hillslope(ranging from 0.073 cm3cm-3 to 0.189 cm3cm3),especially under dry condition.Compared with on TG hillslope,soil moisture have greater spatial variability and spatial autocorrelation on BF hillsope.The controlling factors of soil moisture spatial distribution are also different on these two land types.The dominated controlling factors of soil moisture spatial distribution on BF hillslope are slope,silt.While on TG hillslope,clay,elevation,slope and topographic wetness index(TWI)control the spatial distribution of soil moisture.The influence of topographic features on the spatial distribution of soil moisture are more significant under wetter condition and at deeper soil.(2)The two-dimensional hillslope soil water flow model(Hydrus-2D)and high spatio-temporal resolution soil moisture distribution model were established to describe the features of two-dimensional soil water movement process and soil moisture spatial distribution.The simulation precision of Hydrus-2D was evaluated by Nash-Sutcliffe(NSE).The maximum NSE value is 0.681 and the minimum is 0.502.The NSE values at 10-and 30-cm soil depths at the top of TG hillslope are 0.612,0.573,respectively,while the NSE values at the foot are 0.681 and 0.654 at 10-and 30-cm depths,respectively.The NSE values at 10-and 30-cm depth at the top of BF hillslope are 0.629,0.602,respectively,while at the foot of hillslope are 0.647.0.502 at 10-and 30cm soil depths,respectively.The NSE values of high spatial-temporal resolution hillslope soil moisture distribution model on TG and BF hillslope are all above 0.77.The NSE at 10-and 30-cm soil depth on TG hillslope are 0.854,0.803,respectively,and the NSE at 10-and 30-cm soil depth on BF hillslope are 0.848,0.775,respectively.(3)The subsurface flow flux on TG hillslope is much larger than that on BF hillslope and more sensitive to precipitation and soil texture.The two-dimensional soil water movement on TG and BF hillslopes were simulated by HYDRUS-2D.Results shown that subsurface flow flux on TG hillslope(359.61 mm)is 5.5 times larger than that on BF hillslope(65.35 mm)and intensively occurs in April and July.Moreover,analysis on the subsurface flow fluxes under multi-integrated scenarios of precipitation,soil texture and slope on TG and BF hillslope suggested that the sensitivity of subsurface flow to precipitation,soil texture and slope is larger on TG hillslope(32.82%?90.80%)than on BF hillsope(24.86%?70.53%).Precipitation has great influence on subsurface flow and extreme precipitation events with short duration and high intensity can significant increase subsurface flow(e.g.,subsurface flow increased about 47.25%on TG hillslope).Subsurface flow flux in loamy sand is approximately two times larger than that in other soil textural.Subsurface flow flux increases along with increasing of slope,subsurface flow flux on medium slope is about 1.18 times larger than that on gentle slope.In addition,it is noted that antecedent soil moisture thresholds exist at approximately 0.18 cm3 cm-3 on TG hillslope and 0.31 cm3 cm-3on BF hillslope.Obvious subsurface flow occurs during relatively wet antecedent conditions,when the soil moisture threshold is exceeded.Overall,the land use change from BF to TG and future extreme precipitation events will promote surface water eutrophication in Taihu Lake Basin.(4)The hot-spots distribution of soil moisture variation and their impact factors are different on TG and BF hillslopes.The active level of soil moisture is shown to change over time.Comparing the distribution of hot spots and hot moments of soil moisture movement in two types of land,the study found that at the depth of 10 cm,the active level of soil moisture is higher on BF hillslope while at the depth of 30 cm,soil moisture is more active on TG hillslope,which can be explained by the fact that there is more subsurface flow flux on TG hillslope.The spatial variation of soil moisture active levels is mainly controlled by soil properties(known as particle size distribution).More sands in 10-cm depth,and more clays in 30-cm depth both lead to more active soil moisture.The variation rates of soil moisture in two types of lands change with seasons,shown as highly active in spring and summer,moderately active in autumn and lowly active in winter.Though there are monthly differences that active level of soil moisture is highly related to precipitation and temperature change.(5)Soils on TG hillslope has lower water holding capacity,and produce much more subsurface flow compared with that on BF hillslope,and is more sensitive to environmental factors than that on BF hillslope.During recent years,more bamboo forests are changed into tea garden in the local hilly areas,which apparently increase the subsurface flow and thus the risk of nutrients run-off increase,aggravate the problem of eutrophication.Therefore,it is necessary to control the land type of tea garden in a certain scale,implement appropriate conservation tillage and improve water use efficiency on hillslope.Moreover,to realize the sustainable development of land use on hillslope,subsection land-use planning should be conducted to optimize the combination of different lands and reduce the regional ecological risks caused by land use change.