Characteristics and change of surface depression storage on sloping land with different tillage practices

Abstract: Surface depression storage, which is enhanced by various tillage practices, is an important hydrological element in the sloping land. Because surface depression storage is related to overland flow production and soil infiltration capacity, therefore, it is also one of the important factors to influence soil and water losses in the sloping land. The objective of this study was to measure the changes in surface depression storages and to analyze the characteristics of surface depression storage in the sloping land with 3 tillage practices, i.e. shallow hoeing, deep hoeing and contour ploughing. These are tillage practices commonly occurring on the Loess Plateau of China. A smooth surface served as the control measure. The tillage practices were simulated in soil box with a length of 2 m, a width of 1 m and a depth of 0.5 m. A total of 5 slope gradients were used for rainfall application in this study. Soil surface roughness of sloping land was measured and calculated using a chain method. The rainfall application was conducted using a simulator with 4 spray nozzles. The nozzles were mounted on the position of 15 m above the ground. The rainfall intensity was 90 mm/h, and the rainfall duration was 60 min. In general, surface depression storage is quantified by the maximum depressional storage, which can be calculated using various empirical equations containing roughness indices or be estimated using digital techniques. In this study, surface depression storage was measured using a field measurement method. The results showed that surface depression storages in the sloping land with tillage practices were more than 4 times that on the smooth surfaces. For the different tillage practices, the surface depression storage differed and the order of surface depression storages was contour ploughing > deep hoeing > shallow hoeing > smooth surface. The surface depression storage decreased with the increasing cumulative rainfall. For examples, the surface depression storage decreased by 41%, 28% and 15% for contour ploughing, deep hoeing and shallow hoeing treatments respectively after a successional rainfall event of 90 mm compared to the initial values before the rain. Moreover, the slope steepness of the sloping land also affected the amount of depressional storage. Regression analysis showed that the changes in surface depression storage with the slope steepness of the sloping land could be described by a power function (R2 > 0.70) for the shallow hoeing, deep hoeing, contour ploughing and smooth surface treatments. Once the slope steepness was more than 15°, the change rate of depression storage markedly decreased and then remained in a stable state, implying that slope had a critical role on the tilled surface. Beyond the critical slope steepness, the differences in change characteristics of depression storage between smooth surface and tilled surfaces declined fast. The changes in surface depression storage could be predicted by the ratio of soil surface roughness before and after rainfall. Overall, the surface depression is important characteristic in the sloping land. Tillage practices can increase the amount of water stored in surface depressions. However, both accumulated rainfall and slope gradient lead to the decrease of actual surface depression storage under some conditions. The results provide a mechanistic understanding on how tillage affects surface depression storage.