Soil Water Dynamics And Hydrological Relation Of Cropland And Shelterbelt In The Oasis Zone Of The Heihe River

Water resources are scarce in the middle reaches of the Heihe Oasis Region, Northwest China, but water consumption continue to increase, which makes more acute contradiction between water supply and demand. In order to eased the tension of water resources, we studied the water consumption pattern of different landscape units, proposed the optimized irrigation schemeto improve water use efficiency. This paper selected the farmland and shelterbelts which consumed the largest amount of water, quantified the water movement and water exchange of farmland and shelterbelt by the Hydrus-1D model, and proposedsoil improvement of the clay layerandoptimized irrigation system. The results are outlined as follow:(1) In the Infiltration process of farmland and shelterbelts, the infiltration rate decreasedgradually, and finally became stable. The infiltration rate decreasedsignificantlywhen water flow reached to the clay layer;For the drainage process, soil water holding capacitydecreaseddramatically from soil surface to deeper layer. Therefore, the claylayer had significant inhibition effect on water infiltration and drainage. The statistical analysis indicated that root mean square error(RMSE) and the relative error(RE) between the measured and estimated soil water content and soil water storage were small. HYDRUS-1D model had a high precisesimulation of water vertical transport inthe oasis soil.(2) The soil water content of the different soil layers under maize cropping system exhibited the highest temporal stability among the three land use types. The soil water content measured at the MTSLs of each soil layer under differentland use type can finely predict their mean soil water content. Soil water relations only occurredbetween adjacent land use types but not between non-adjacent land use types. In the upper soil layer(0-190 cm), soil water relations were mainly affected by the water uptakeof the shelterbelt tree root that extended to the adjacent cropland. In the lower soil layer(190-260 cm), the soil water relations among the three land use types were due to groundwater recharge, which caused by crop irrigation that raised the water table to a level at which it could replenish this soil layer.(3) The trees roots were mainly distributed within 5 m of both sides of an irrigation channel between the first and second tree rows. The maximum distance that trees extended their fine roots horizontally was about 18 m from the shelterbelt. In 2-m soil profiles, fine roots were mainly distributed in the 1.4-2.0 m and 0-0.4 m layers depending on the available water sources. A positive relationship was observed between soil water and fine root mass density. Trees grown near the cropland-shelterbelt border exploited water from cropland irrigation and irrigation channel leakage, greatly enhancing their transpiration. During the growing season of 2013, the mean total transpiration of trees grown far away from the border(10.75 and 17.45 m) was 216.9 mm, whereasthe amounts were 670.1 and 488.7 mmfor trees grown near the border(0.85 and 6.30 m), respectively. If the trees were assumed to absorb the same amount of water from soil, rainfall and groundwater sources, then irrigation water sources provided 67.6% and 55.6% of the waterto meetthe transpiration requirements of the trees closest to the border.(4)The fine root maximum depth of the shelterbeltswith different tree specieswas basically the same, but the spatial distributions were significantly different. Soil water distributionof farmland and shelterbelts wasobviously different. Because the measured and simulated soil moisture had a high degree of consistencyin different depths, the HYDRUS-1D model had a high precisesimulation of water vertical transport inthe oasis soil.Because offrequentfarmland irrigation, large amounts water appeareddeeplayerleakage. Due to water shortages,the water consumption ofshelterbelt was smaller.(5) During the growing season of shelterbelt, the level exchange of soil moisture was mainly a one-waypattern from farmland to shelterbelts. The exchange of soil moisturewas 1465.1mm between spring wheat and shelterbelt, of which 74.26% and 25.74%come from trees transpiration and lateral seepage, respectively. The exchange of soil moisturewas 2195.2 mm between maizeand shelterbelt, of which 76.20% and 23.80% came from trees transpiration and lateral seepage, respectively. The exchange groundwater flowwas two-way pattern, but the general flowing trend is from the wheat field in the south to the maize field in the north. Groundwater flow rate was closely related to farmland irrigation.(6) Compared to conventional irrigation, the optimized irrigationof farmlandwithclay layersharplyreducedthenumber of times and total amount of irrigation. Combined with factors asplow land, crop rotation and economic cost, the best depthfor the clay layer was located at 30-40 cm of the soil profile.After applyingoptimization irrigationtothree kinds ofshelterbelts, water stress was eased.