Quantitative study on the slope variation and scale effect of three types of soil water holding capacity in semi-arid loess hilly region

To explore a precise and convenient method for the “from plot to slope” scale conversion of soil water holding capacity, three typical slopes with the upper part of reforested fields and the lower part of terraced fields were selected in the loess region of southern Ningxia. Along the slope position gradient, five plots were established on each slope to measure the maximum, capillary and field water holding capacities of the soil at depths of 0-200 cm, and their spatial variation characteristics on the slope were analyzed. Besides, the scale effect was quantitatively described by the change in the slope sliding average value with the horizontal distance from the slope top, and empirical equations were established between the values of plots at different positions and the slope average value. The results showed the slope-aspect differences were small in three soil water holding capacities, whereas evident differences existed among slope positions and across scales. On the east slope, the average values of soil maximum water holding capacity, capillary water holding capacity and field water holding capacity were 43.56%, 34.54% and 29.41%, respectively, which were close to those on the west and south slope. The relative errors were between 0.17% and 3.71%. As the horizontal distance from the slope top increased, the soil water holding capacity generally showed a decreasing trend, and the slope position where the maximum values occurred varied. The empirical equations were constructed between the soil water holding capacity ratio (plot v.s. slope) and the horizontal slope length (R²>0.85, P<0.0001), with the relative errors of less than 5.09% for estimating the slope average value. Therefore the precise and convenient up-scaling of soil water capacity from plots to slopes can be achieved within the study area.