干旱区生态环境知识资源中心

Soil profile has been used to estimate diffuse recharge and land-use change effects in a semiarid rain-fed agricultural area in the Chinese Loess Plateau. Measurements have included Cl, NO3, delta H-2 and delta O-18 content in soil moisture and water chemistry and isotopes (H-3, delta H-2 and delta O-18) in groundwater. The results show that the diffuse recharge rate estimated from chloride mass balance of two soil profiles is 33-56 mm/yr beneath crops, as the main land use of the study area. The infiltration rate is 0.13 to 0.27 m/yr. Due to the limited recharge rate and thick unsaturated zone, it takes 110 to 460 years for annual precipitation to reach the water table, as confirmed by the absence of tritium (<0.3 tritium unit). The nitrate introduced by human activities is mainly distributed in the upper unsaturated zone, and the groundwater nitrate content (0.1 to 9.3 mg/L as NO3) remains at baseline level. Groundwater Cl content and stable isotopic composition (delta H-2 and delta O-18) is lower than that in the upper unsaturated zone, which is possibly related to recent climate change and human activities. The conversion from crops to orchard has resulted in decrease in soil moisture and increase in Cl content, suggesting decrease in deep drainage. This does not appear to be favourable to the groundwater resources, and could accelerate depletion of the soil reservoir in the Chinese Loess Plateau.