EVALUATING THE EFFECT OF TREE PLANTATIONS on UNSATURATED ZONE FLOW WITH A PAIRED PROFILE MATRIC POTENTIAL MONITORING STUDY, LOESS PLATEAU, CHINA

Soil conservation measures taken to mitigate soil losses from erosion are globally widespread environmental changes that have the potential to affect local and regional water cycles. China's Loess Plateau region has among the highest agricultural soil erosion rates in the world because of steep terrain, highly erodible soils and intensive agriculture. The region has been subject to intensive soil conservation over the past 40 years, approximately half of which has involved the introduction of trees and shrubs in order to reduce runoff from steeply-sloping terrain. Resulting reductions in regional streamflows and plant-available soil moisture have been documented. We present evidence that the plantations also affect unsaturated zone flow conditions to several meters depth. Matric potential profiles beneath adjacent plots with differing land uses (plantation trees versus rain-fed wheat) were monitored on a daily basis for one year. The technique uses commerically available heat dissipation sensors installed in boreholes of 8 m depth. Results show that dry moisture conditions beneath the plantation site were persistent throughout the monitoring period including the summer monsoon rainy season. In contrast, the rain-fed wheat plot experiences frequent wetting fronts. The results suggest that no deep drainage which could lead to groundwater recharge is occuring beneath the plantation. The study emphasizes the potential for negative impacts of some soil conservation approaches on water resources, and that major changes to the Loess Plateau's subsurface water cycle may be a consequence of land use practices of the past several decades.