Ecological water requirement of natural grassland vegetation in the Otog Banner of Inner Monglia

Natural grasslands have become essential to the development of pastoral areas, due mainly to the area of natural grassland accounts for more than 90% of the available grasslands in China. Furthermore, pastoral areas have great potential for ecological barriers and national food security. However, the grassland resources are rapidly decreasing in a seriously damaged ecosystem, particularly with the climate of global warming and regional drying. Vegetation restoration can be an important measure for ecosystem restoration on the natural grasslands. Therefore,water is the dominant factor to control the growth of vegetation in the regional drying grasslands. The Ecological Water Requirement (EWR) of natural grassland can be defined as the amount of water required to maintain normal growth of vegetation in the special ecosystems. It is necessary to coordinate the water use relationship between economic development and ecological environment in pastoral areas, thereby achieving the rational allocation of water resources. In this study, a Penman-Monteith formula was utilized to calculate the evapotranspiration of reference crops in the Otog Banner of northern China from 1960 to 2016. A Mann-Kendall test was conducted to evaluate changing trend of evapotranspiration with time. A remote sensing inversion of soil moisture content was also used to calculate the limiting coefficient of soil moisture. An investigation was made to count the coverage rate of vegetation in the natural grassland and the growth period of dominant plants in different grassland types. A piecewise single average crop coefficient method (FAO-56) was used to calculate the crop coefficient in the grasslands. The reference evapotranspiration contributed to obtaining the limiting coefficient of soil water, the crop coefficient, the requirement and shortage of ecological water for various grassland types in different regions. A tradeoff was gained to clarifythe relationship between ecological water demand and rainfall resources for the different types of natural grassland. The results are as follows: 1) The grassland ecosystem in the OtogBanner was in a state of water deficit as a whole, especially in the representative area of Huinong station, where the water deficit was more than 50% in a dry year. 2) The annual EWR of the whole banner was 2.94×109 m3 in wet years, 3.07×109 m3 in normal water years, and 3.10×109 m3 in dry years. The annual ecological water shortage was 2.72×108m3 in wet years, 5.00×108 m3 in normal years, and 1.15×109 m3 in dry years. The EWRs of grasslands were ranked from large to small: the temperate desert steppe, temperate steppe desert, lowland meadow, temperate desert, and temperate steppe. 3) There was an unbalanced state of EWR for all kinds of grassland vegetation in the Otog Banner. Specifically, the serious water shortage occurred in May, followed by August, April, July, and September, whereas, there was a surplus state of EWR in March, June, and October, where the largest surplus EWR occurred in June. A suggestion was made to store the rainwater in surplus months, thereby solving the spatiotemporal unbalance of rainwater resources in different seasons. These recommendations can greatly contribute to maintaining the healthy and sustainable development of grassland ecology.