Soil moisture response to rainfall on the Chinese Loess Plateau after a long‐term vegetation rehabilitation

Abstract

The Chinese Loess Plateau (CLP) is a unique Critical Zone with deep loess deposits, where soil moisture is primarily replenished by seasonal monsoon rainfall. However, the role of vegetation, coupled with complex topography, on rainwater infiltration on the CLP, especially after long‐term revegetation for controlling erosion, is inadequately quantified. Over the growing season of 2016, we monitored soil moisture at the 30‐min interval at 5 depths (10, 20, 40, 60, and 100 cm) in an afforested catchment and a nearby catchment with natural regrowth of grasses. Two monitoring sites were established in each catchment, one in the downhill gully and the other in the uphill slope. We found that vegetation, topography, and rainfall attributes together determined rainwater infiltration and soil moisture replenishment. An accumulated rainfall amount of 9 mm was required to trigger soil moisture response at 10‐cm depth at the 2 grassland sites and the forestland uphill‐slope site whereas 14 mm of rainfall was required for the forestland gully site covered by dense undergrowth and trees. Rainfall events with larger sums and higher peak intensities permitted rainwater infiltration to deeper soil depths. However, no rain recharged soil moisture to 100‐cm depth during the monitoring period. The forestland uphill‐slope site showed the deepest wetting depth (up to 60‐cm depth), fastest wetting‐front velocity (up to 4 cm/hr below 10‐cm depth), and the most significant soil moisture increase (up to 15% cm 3 cm−3 increase at 10‐cm depth) after rainfall in the growing season. The grassland gully site had the highest soil water storage, whereas soil moisture was depleted the most at the forestland gully site. Findings of this study reveal the transient dynamics of soil moisture after rainfall on the CLP, which signifies the role of revegetation on rainwater infiltration in the loess Critical Zone.

Effects of Deforestation on Water Flow in the Vadose Zone

“…The observed differences were likely related to wetter conditions (e.g., less bypass flow due to hydrophobicity, larger hydraulic conductivity of the matrix) in the deforested part of the catchment (Figure 3) and the larger amount of direct rainfall (no interception). Whereas the results presented here showed clear differences in sequential flow occurrence, several earlier studies have reported clear differences in preferential flow occurrence that were related to different vegetation types [27,39,40,49]. Alaoui et al [27] found preferential flow both in forest and in grassland soil, but dye patterns suggested stronger interactions between macropores and matrix for forest soils.…”

“…The macropores in the grassland soils, on the other hand, showed little interaction with the surroundings. Jin et al [39] and Demand et al [40] found clear differences in preferential flow occurrence between grassland and forested sites. Zhao et al [49] also observed more preferential flow occurrence for a forest site as compared to a grassland site.…”

“…The macropores in the grassland soils, on the other hand, showed little interaction with the surroundings. Jin et al [39] and Demand et al [40] found Overall, clear changes in sequential and no flow occurrence were observed, which could directly be linked to the partial deforestation in the Wüstebach catchment. The observed differences were likely related to wetter conditions (e.g., less bypass flow due to hydrophobicity, larger hydraulic conductivity of the matrix) in the deforested part of the catchment (Figure 3) and the larger amount of direct rainfall (no interception).…”

“…In recent years, soil moisture response time analysis has been introduced, and this type of analysis offers more quantitative spatiotemporal information on the occurrence of preferential and piston flow in the subsurface [34][35][36][37][38][39][40]. The aim of this study is to use a sensor response time analysis to investigate changes in subsurface flow behavior related to partial deforestation in a small headwater catchment in Germany.…”

“…The aim of this study is to use a sensor response time analysis to investigate changes in subsurface flow behavior related to partial deforestation in a small headwater catchment in Germany. Two recent studies by Jin et al [39] and Demand et al [40] already used such an analysis to characterize differences in flow behavior related to vegetation. Both studies showed that the occurrence of preferential flow and the infiltration capacity can be significantly larger in forests as compared to grassland sites.…”