Use of the SHAW model to assess soil water recovery after apple trees in the gully region of the Loess Plateau, China

Because of the high income resulting from apple production, apple orchards have widely spread since the 1980's in the gully region of Loess Plateau of China. Compared to winter wheat, a major crop in this region, apple trees deplete most of the available water in the 010 in soil profile after 30 years, resulting in a decline of apple yield and profitability. Apple trees older than 30 years are often cut, and orchards reconverted to winter wheat, but wheat planted after apple trees suffers from low yield, and groundwater from low recharge rate due to reduced soil water content. The objective of this study was to determine, with the one-dimensional simulation model Simultaneous Heat and Water Transfer (SHAW), the time required for soil water to recover after apple tree cutting. Recovery time is defined as the amount of time required for a field just cut of its apple trees to recover to the same soil water content of a field that has always been cropped to wheat. SHAW soil and crop parameters were calibrated (1984-1989) and validated (1989-1994) using measured soil water contents in bare soil and winter wheat plots. Using calibrated soil and crop parameters, soil water content variations in the 0-10 in soil profile was simulated under winter wheat conditions for forty randomly generated climatic sequences based on observed 48-year historical data (1957-2004). Results show that the recovery time varies from 6.S to 19.S years, with an average of 13.7 years, for the 0-10 in soil profile, and from 4.4 to 8.4 years, with an average of 7.3 years, for upper 0-3 in soil profile. Due to lower soil water content, the mean groundwater recharge rate would be 9.3 mm year(-1) in orchard cut plots during the recovery period, compared to 18.3 mm year(-1) for land that had always been in winter wheat. Considering the long-term effects of apple orchards on land productivity and groundwater recharge, areas in apple orchards should be restricted to maintain sustainable agriculture in this region. (c) 2006 Elsevier B.V. All rights reserved.