Determination for suitable scale of oasis and cultivated land in middle reaches of Heihe River basin

The middle reaches of Heihe River are the essence parts of Heihe River, and its stability is directly related to the sustainable development of economy, society and ecology in this basin. But excessive use of water and soil resources caused by population increase and economic development since the 1960s, has led to ecological environment deterioration and decrease of runoff flowing into lower reaches. In order to relieve water shortage and use water resources efficiently, the State Council approved plan of Water Reallocation of the Heihe River in 1997 to stipulate the water discharging of Zhengyi gorge. Under the stipulation, the flow coming from Yingluo gorge are under different assurance probabilities. As such, how to develop appropriate oasis scale using the limited water resources has become one of the important research subjects of sustainable development in the middle reaches of Heihe river. The research of suitable scale for oasis and cultivated land in arid region can provide the theoretical and technical guidance for sustainable utilization of regional water resources. Choosing Heihe river basin as the study area, hydro-meteorological data during 1954-2012 and land-use/cover data in 2000 and 2011 were used to analyze the variations of high and low flow for surface runoff in the middle reaches of Heihe River. A suitable oasis scale computation model was built based on water-heat balance, which included calculation of evapotranspiration of forest and grassland. According to Egyptian scholar M.M.Abd EI-Ghani's theory, the oasis was divided into three zones based on classification of vegetation. A suitable cultivated land scale was adopted and it combined with the suitable oasis model was used to calculate proportion of cultivated land in oasis. Combining the Markov chain based on GIS with suitable oasis scale model, using land use/cover data to calculate Markovian transition matrix, and then to calculate annual transition probability. Based on land use/cover data in 2011 and annual transition probability, oasis scale in 2015 and 2020 was predicted, and its suitability was evaluated. Results showed that the runoff in middle reach of Heihe river changed during the studied time period. The suitable oasis scale was 3 245-4 868, 2 635-3 953 and 2 422-3 633 km2 in high flow, normal flow and low flow years, respectively. The suitable cultivated land scale was 2 337, 2 004, 1 878 km2 accordingly. For the high flow year in 2011, scale of oasis and cultivated land were 4 035 and 2 366 km2. It was showed that when the cultivated land area exceeded the suitable scale, and the suitable scale of oasis and cultivated land in 2015 would expand to 4 155 and 2 527 km2, respectively. Although suitability index of oasis scale declined to 0.44-0.59, the suitable scale of oasis and cultivated land in year 2020 would expand to 4 304 and 2 719 km2, respectively. When the suitability index of oasis scale decreased 0.02-0.03 from the level in 2015, the scale of cultivated land area exceeded the suitable scale Suitable oasis scale is not only related to available agricultural water resources, also has close relationship with the oasis agriculture water saving level, future research should further consider the influence of water saving level on oasis scale.