Hydrological processes and eco-hydrological effects of farmland-forest-desert transition zone in the middle reaches of Heihe River Basin, Gansu, China

The study of the hydrological processes in the transition zone is important, but more complex compared with the homogenous land use units. A typical farmland-forest-desert transition zone in the Heihe River Basin was selected to study the hydrological processes and eco-hydrological effects among these land use units by monitoring the soil water content (SWC), groundwater level (GWL), and vegetation dynamics. Results showed that the sharp fluctuations of daily SWC and GWL in the farmland and the forest were primarily attributed to the irrigation events (7 and 6 times for the farmland and forest, respectively). The hydrological links among the three land use units were exhibited in three patterns. First, the soil water of the upper soil layer near the interface of two land use units moved from the irrigated land use unit to the non-irrigated one under soil water potential gradients through physical diffusion (the lateral water flow rate was less than 1 cm d(-1)). Second, the water flowed from the irrigated land use unit to the non-irrigated one under GWL gradients through groundwater flow (the lateral groundwater flow rate was less than 10 cm d(-1)). Third, a portion of the soil water in the farmland was utilized by the extended root system of the trees. The water exchange between the farmland and the forest resulting from one irrigation event was 5-30 mm, which caused increased GWLs for 1 week. At the forest-farmland boundary, the impacts of the extended tree roots reduced maize growth and extended 10-15 m into the farmland. By contrast, no obvious impacts were observed at the forest-desert boundary. Irrigating the farmland and the forest separately and reducing the width of the forest by 15-20 m would be more beneficial for irrigation water efficiency. These results would be useful for soil water management in terms of water balance impact on ecological construction and implementation of water-saving agriculture, as well as for optimal design of land-use patterns and efficient protection of oasis ecosystems to preserve limited water resources. (C) 2015 Elsevier B.V. All rights reserved.