Water allocation and water consumption of irrigation agriculture and natural vegetation in the Heihe River watershed, NW China

To implement effective water resource management strategies, a sufficient data basis about the hydrologic situation must be available. An important parameter is the water consumption by the natural environment, e.g., evapotranspiration (ET). This study delivers actual evapotranspiration rates (ET_a) computed on the basis of Landsat TM/ETM+ and MODIS data. Vegetation mappings recorded during a field work campaign in 2012 allowed the correlation of ET rates to certain units of vegetation. The study site is located in the Heihe river basin, Northwest China, where the landscape is characterized by extending land under cultivation along the middle reaches and ecologically valuable Tugai vegetation further downwards. Due to the arid climate, all agriculture depends on irrigation with water taken from the Heihe River. As a result of a massive expansion of irrigated land in the last decades, an imbalance with regard to water allocation has developed. It is characterized by an overexploitation of the water resources in the middle reaches and a strong degeneration of the natural Tugai vegetation along the lower reaches due to water shortage. As a response, a water distribution plan has been adopted to define target amounts of water that shall reach the lower parts of the river. Total values of the ET_a over the whole vegetation period for corn, dates, orchards and grapes amount to 667.7, 762.2, 703.5 and 483.9 mm, respectively. For forest vegetation (ground coverage 80 %) and for shrub vegetation (80–100 % ground coverage), values of 496–530 mm and 177–233 mm were calculated, respectively. Areas with lower ground coverage show significantly lower ET_a values. Spatially, ET_a values decrease from the center towards the border of the oasis and from the middle towards the lower reaches. Agricultural land presents the largest water consumer. The total ET_a values over the vegetation period amount to 2.15 km³ (middle reaches) and 0.28 km³ (lower reaches), respectively. On this basis, a minimum discharge of 0.34 km³ was calculated to maintain the natural vegetation downstream.