Ecohydrology in water-limited environments using quantitative remote sensing – the Heihe River basin (China) case Promotor: Prof. dr. sc. nat. M. E. Schaepman Hoogleraar Geo-informatiekunde mbav Remote Sensing, Wageningen Universiteit (Nederland) Co-promotoren: Dr. ir. J. G. P. W. Clevers Universitair hoofddocent bij het Laboratorium voor Geo-informatiekunde en Remote Sensing, Wageningen Universiteit (Nederland) Prof. dr. Z. Su Hoogleraar Spatial Hydrology and Water Resources Management, International Institute for Geo-Information Science and Earth Observation – ITC, Enschede (Nederland) Promotiecommissie: Prof. dr. ir. R. Uijlenhoet Wageningen Universiteit (Nederland) Prof. dr. ir. M.F.P. Bierkens Universiteit Utrecht (Nederland) Prof. dr. ir. N.C. van de Giesen TU Delft (Nederland) Dr. L. Jia Alterra, Wageningen UR (Nederland) Dit onderzoek is uitgevoerd binnen de C. T. de Wit onderzoekschool Production Ecology and Resource Conservation (PE&RC) Ecohydrology in water-limited environments using quantitative remote sensing – the Heihe River basin (China) case Xiaomei Jin Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit Prof. dr. M. J. Kropff in het openbaar te verdedigen op woensdag 25 februari 2009 des namiddags te vier uur in de Aula Xiaomei Jin, 2009 Ecohydrology in water-limited environments using quantitative remote sensing – the Heihe River basin (China) case PhD Thesis, Wageningen University, Wageningen, the Netherlands With summaries in English, Dutch and Chinese ISBN 978-90-8585-290-2 Table of Contents CHAPTER 1 Introduction …………………………………………………………………..1 CHAPTER 2 Quantification of spatial distribution of vegetation in the Qilian Mountain area with MODIS NDVI ………………………………………....9 CHAPTER 3 Impact and consequences of evapotranspiration changes on water resources availability in the arid Zhangye basin (China) ………………...27 CHAPTER 4 Runoff hysteresis effects of the Heihe River on the vegetation cover in the Ejina oasis (China) ………………………………………………….. 45 CHAPTER 5 Effects of groundwater depth on vegetation growth in the Ejina area (China) …………………………………………………………………59 CHAPTER 6 Synthesis …………………………………………………………………….77 REFERENCES……………………………………………………………………………… 89 SUMMARIES Summary…………………………………………………………………….107 Samenvatting………………………………………………………………...110 内容概要…………………………………………………………………….113 Acknowledgements………………………………………………………………………....115 Curriculum Vitae…………………………………………………………………………... .117 List of Publications………………………………………………………………………….118 PE&RC PhD Education Certificate………………………………………………………... .121 Introduction CHAPTER 1 Introduction 1 Introduction 1.1 Background Arid, semiarid and subhumid regions occupy approximately 50% of the global land surface (Parsons and Abrahams, 1994). These regions and their environments are considered to be water limited because annual precipitation is typically less than the annual potential evapotranspiration (Guswa et al., 2004). Although variable with respect to physiography, geology and soils, these environments are often sensitive and vulnerable because of low and highly variable precipitation, limited water resources and sparse vegetation. The environmental changes occurring over vast areas in these arid regions include land desertification, groundwater depletion, salinization, and soil erosion (De Fries et al., 2004), amongst others. These environmental changes increasingly affect human societies and have a growing influence on global biogeochemical cycles (Schlesinger et al., 1990; Bonan, 2002). Vegetation, both native and cultivated, strongly influences the environment and is influenced itself by the environment (Sabins, 1996). The vegetation is an environmental indicator in water-limited ecosystems and is often linked to both the causes and consequences of arid land degradation. The role of vegetation in the dynamics of soil moisture, runoff, and streamflow has been acknowledged to be very important (Wilcox et al., 1997, 2003b; Newman et al., 1998, 2004; Neave and Abrahams, 2002; Porporato et al., 2002; Ridolfi et al., 2003; Fernandez-Illescas and Rodriguez-Iturbe, 2004; Cayrol et al., 2000; Kerkhoff et al., 2004b). Understanding the influence of vegetation on hydrological changes is part of the foundational basis of ecohydrology (Newman et al., 2006). Therefore, studies on quantifying the relationship between the vegetation and water resources represent a critical step in developing advanced ecohydrological approaches, supporting resource management and environmental change. The above-mentioned arid regions occupy a vast area in north-western China, covering about 2.5 million km² or one-quarter of the Chinese territory. In these regions, mean annual rainfall is less than 250 mm, and even decreasing towards the western plains (50-150 mm) and the Ejina area (less than 40 mm). The annual potential evaporation is in general more than 1,400 mm, and can exceed 2,000-3,000 mm in the desert areas. Because of the arid climate, about 70% of the total arid regions are unusable for human activities, such as sandy deserts, gravel deserts, and other sorts of xeric shrublands. During recent years, the recession of the vegetated parts of the ecosystems appeared to be extensive in Northwest China. They caused a series of environmental problems, like the shrinking of the oasis area and land desertification resulting in increasing sources of sandstorms. Water resources are the essential factor influencing the vegetation variability (Dawson, 1993; Burgess et al., 1998; Caldwell et al., 1998; Brooks et al., 2002; Zou et al., 2005; Santanello et al., 2007). In the northwestern arid area of China, all the oases are fed by surface rivers and their extent has a close relationship with runoff of the river and the groundwater depth. However, due to little population density, inconvenient transportation and shortage of available long-term monitoring data, traditional 3 Chapter 1 methods performing qualitative ecohydrological analysis that usually employ point observations and are only representative for local scales, cannot be extended to large areas. The use of remote sensing can provide continuous and representative measurements of several relevant physical parameters at scales from point to continent. These methods are still used in a limited fashion in hydrology for a quantitative assessment of the eco-environmental changes in China (Li et al., 2001; Lu et al., 2003; Guo and Cheng, 2004; Kang, et al., 2007). The purpose of this study is to develop a method to quantitatively assess the eco-environmental changes using remote sensing methods and applying it to ecohydrological applications in China. The Heihe River basin, located in the middle of the Hexi Corridor of the Gansu Province, is one of the two largest inland river basins in China. Its watershed covers an area of 14.3 ×10 4 m2 and the upper, middle and lower reaches of the Heihe River stretch from the middle of the Hexi corridor to the western Inner Mongolia Municipality. In the southern part of the Heihe River basin the Qilian Mountains are located representing the upstream area, which are steep mountains with an altitude ranging from 3000 m to 5000 m above sea level. Due to the cold climate and the sufficient precipitation, the runoff generated from this area is the main source of the surface water and groundwater for the Heihe River basin, and finally ends in two terminal lakes of the Ejina Oasis (the downstream area), namely, West Juyan Lake and East Juyan Lake. The middle stream area, called the Zhangye basin, is a very important agricultural area in northwest China. With growing population and farmland expansion in the middle stream area, the water consumption has increased gradually and most water is nowadays used for irrigation. This is causing a decrease of incoming water in the downstream area resulting in a serious recession of the eco-environment in that region. The Chinese government puts significant importance on improving the eco-environment of the downstream area by balancing the water consumption and has therefore implemented a new policy for the allocation of available water resources. An applicable method for a quantitative analysis of the eco-environmental changes as well as providing scientific evidence for protecting and improving the eco-environment in these Chinese Northwestern arid regions is the final goal of this study. 1.2 Remote sensing in ecohydrology Remote sensing has long been suggested as being a time- and cost-efficient method for monitoring changes in arid environments. It can detect and monitor landscape change and degradation in arid and semiarid regions. The use of remote sensing for deriving process- relevant environmental information from optical remote sensing data in arid areas is highlighted in several environmental degradation studies (Okin and Roberts, 2004; Bai et al., 4 Introduction 2008). Therefore, using remote sensing methods to understand eco-environmental changes has emerged to be a current research topic of wide interest. The complexity and heterogeneity of hydrological processes exist over a wide range of scales in space and time. Traditional techniques measuring hydrological variables rely on point sensors collecting information which is assumed to be representative for large areas. However, this approach is not particularly helpful in complex or heterogeneous environments where the point measurements cannot be assumed to represent large areas. The surface- atmosphere interface is an example of a system that is highly variable