Soil Salinization And Its Dynamic Relationship With Exchange Water Flux Under Mulched Drip Irrigation In An Oasis

Agriculture in Xinjiang province relies greatly on irrigation due to limitedprecipitation and water resources. Water scarcity is becoming more and more acute inrecent years associated with both population growth and the extension of irrigatedfarmland, which further aggravates the water shortage situation and ecosystemdegradation. Meanwhile, soil salinization induced by large scale irrigation has severelythreatened the sustainable developments of these oases. Water-saving irrigation, as oneeffective approach to mediate water conflicts between human and nature, has beenpopularized in arid regions. However, it remains unclear that how water-savingirrigation alters overall water balance dynamics and affects soil salt migration.Therefore, based on the data obtained by the field experiments which were implementedin the Tsinghua University–Korla Oasis Eco-hydrology Experimental Research Station,the soil salinization and its dynamic relationship with exchange water flux undermulched drip irrigation were analyzed and discussed in this thesis. The insights gainedfrom this study can be applied to improve the water management in the arid andsemi-arid areas to maintain the sustainable development.The intra-annual and annual patterns of soil salt distribution and accumulationwere explored based on40000soil samples collected from2008to2013under mulcheddrip irrigation. The results showed that within50cm soil depth, the soil salt remarkablyincreased during the seedling stage. Then the salt decreased in the beginning due toleaching when the drip irrigation started, and later it accumulated and increased. At theend of the drip irrigation stage, the soil salt increased by25%within the50cm soildepth comparing with the original soil profile at the beginning of seedling stage. Ingeneral, the zone below drip pipe obtained the lowest salinity level and the salt mainlyaccumulated in the inter-film zone. The effect of salt leaching was obvious during bothwinter and spring flush. The leaching factor of soil salt was approximately10%in thespring flush. According to the soil salt leaching during non-growth period in arid area,the soil salt will decrease by5%or more within the whole year and soil salinizationtrend can be controlled when mulched drip irrigation is adopted in the growth period. Otherwise, the soil salt will increase by14%without the flush implemented innon-growth period.Three measurement techniques at different scales were used to obtainevapotranspiration: photosynthesis system (leaf scale), sap flow (plant scale), and eddycovariance (field scale). To upscale the evapotranspiration from the leaf to plant scale,an approach that incorporated the canopy structure and the relationships between sunlitand shaded leaves was proposed. To upscale the evapotranspiration from the plant tofield scale, an approach based on the transpiration per unit leaf area was adopted andmodified to incorporate the temporal variability in the relationship between leaf areasand stem diameter. The results indicated that the proposed upscaling approaches werereasonable and valid, and the evapotranspiration components were analyzed for cottonfield. Based on the evapotranspiration measurements, the water balance analysis wasclosed and the exchange water flux was directly quantified at the field scale. Thedynamic relationship between soil salinization and exchange water flux was explored.This dynamic relationship suggested that under mulched drip irrigation the downwardexchange water flux was reduced during drip irrigation stage and a new form ofsecondary salinization may be introduced unwittingly due to the lack of water for saltleaching. Meanwhile, the upward flux was trivial due to the moderate groundwatertable depth and traditional secondary salinization caused by intense phreatic evaporation(fed by upward exchange water flux) was alleviated.The water use efficiency (WUE) of mulched drip irrigation was3.1times the valueof flood irrigation. Mulched drip irrigation significantly enhances the WUE at fieldscale. However, to avoid the irrigation efficiency paradox at regional scale, theagriculture field expansion should be strictly controlled when the water-savingirrigation is popularized in the oases in the future.