Multi-scale Modeling Of The Agro-Eco-Hydrological Processes In Irrigation District

Water scarcity,soil salinization and land degradation are the main factors threatening the sustainability of arid agro-ecosystems.Fully understanding the hydrological processes and solute transport in arid irrigated areas is of great significance to improve agricultural water use and maintain ecological security.Land fragmentation and heterogeneous crop distribution on small fields are remarkable characters of agriculture in China and some other developing countries.They also have significant effects on agro-hydrological processes.In this study,the Hetao Irrigation District(Hetao)located in the upper Yellow River basin was taken as a case study area to explore the hydrological complexities in irrigated agro-ecosystems with fragmented land cover types.The main research contents and conclusions are as follows:(1)A sub-irrigation district(Jiyuan)in Hetao was selected as a typical case study region.Field observations were conducted in Jiyuan as well as a small irrigation system(YCA)within it during 2012-2016.Observation items including soil water and salt,soil physical properties,plant growth,irrigation and drainage and groundwater.Based on the observation and statistical data,land use and land cover,cropping pattern,soil water and salt distribution and groundwater depth were analysed through remote sensing and GIS.Results showed that cropland and natural land were the main land use type and accounted for more than 90%of Jiyuan.Various kinds of crops grown in the cropland with maize and sunflower occupied more than 80%.The main soil type in the study area was silt loam.The average annual precipitation was 150 mm,in which nearly 90%occurred during the growing season(May to September).The area had 6-7 water diversions during a year.The total water diversion averaged 580 mm with the autumn irrigation accounted for 40%.The adjustment of cropping pattern was changing the water allocation schedule,yet the total water diversion amount was almost the same.Groundwater level declined from south to north,similar with the terrain trend.The groundwater depth cyclically varied from 0 to 3 m during a year.The soil water content of the 0-100cm soil profile in late April was high(above 0.2 cm3cm-3,averaged 0.4 cm3cm-3)and showed close relationship with groundwater depth.The natural land had very high soil salt content and low canopy coverage.(2)Based on the water balance calculation of YCA,specific yield and the evapotranspiration of some typical fields were obtained.A 13-year(2001-2013)water balance calculation was carried out to quantify the water budgets of different land use types in Jiyuan.The effects of shallow groundwater on water and salt exchange among different land use patterns were analyzed.Results indicated the shallow groundwater system played a significant role in water storage and supply,water and salt redistribution,and the salt accumulation and drainage in Jiyuan.About 36%of the total applied water should firstly be stored in shallow groundwater,and then be redistributed.After redistribution,63%of the total diverted water was consumed by cropland evapotranspiration(ET),20%was consumed by natural land ET and the rest was discharged through drainage or groundwater exploitation.Finally,67%of the introduced salt accumulated in natural land and the rest was drained away from the region(3)Considering the shallow groundwater,film mulching and large patches of natural vegetation,the model HYDRUS-1D coupled with the FAO-56 dual crop coefficient approach(dualKc),was applied to simulate eco-hydrological processes in the study area.The model was calibrated and validated in three crop fields and one natural field using the field observation data in 2013 and 2012.Results showed that model simulations were in good agreement with the observed values.Excessive irrigation produced large deep percolation,averaging 40%of the water applied during the simulation period(May to September).Under conditions of field crop fragmented diversity,diverse irrigation schedules and shallow water tables,the percolated water in irrigated fields was significantly reused by capillary rise in the same areas and in nearby non-irrigated fields due to the lateral migration of groundwater.This explained the large capillary rise,accounting for 23-53%of total evapotranspiration for different crop fields,and 75-82%for natural land.At the same time,soil evaporation reached about 22-55%of the total evapotranspiration in cropland and 50%in natural land,which aggravated the salt accumulation to the surface layer.Both crops and the natural vegetation were primarily affected by salt stress not water deficit,as the root zone water content was relatively high during the season.(4)Based on the previous studies,the HYDRUS-dualKc model was used to predict the changes in transpiration,evaporation and salt accumulation considering scenarios with various GWDs and irrigation strategies.The integration results from the three crops and the natural vegetation simulation scenarios were analysed and an optimum GWD of between 1.7-2.3 m for the crop fields and 1.4-2.0 m for the natural land were recommended.Some additional irrigation strategies for watermelon and tamarisk were also suggested.(5)The HYDRUS-dualKc model was used in a distributed manner with the assistance of GIS,remote sensing data and field observation network in Jiyuan.The model application was successful as shown by the goodness-of-fit indicators relative to simulating soil water content,soil salinity concentration and evapotranspiration(ETa).The hydrological complexities in agro-ecosystems with fragmented land cover types and various management strategies were assessed based on the model simulation and water balance calculation.Results showed that the groundwater recharge zone and discharge zone occurred randomly and characterized by interlaced distribution,which is favourable to the percolation water reuse.However,this local groundwater exchange also weakened the spatial trend of water and salt transport in the regional scale and was unfavourable to the water and salt drainage.Through lateral groundwater exchange,the cropland functioned as a stable recharge zone while the natural land functioned as a discharge zone during the growing season.As a result,21%of the total water diverted to Jiyuan migrated to natural lands through groundwater due to field percolation and canal seepage,along with 50-75%of the total salt introduced.The fragmented land covers and complex hydrological processes bring many challenges to remote sensing,model simulation and the application of modern agricultural technologies.