Contribution of recycled moisture to local precipitation in the inland Heihe River Basin

Recycled moisture contributed by continental evaporation and transpiration plays an important role in regulating the hydrological processes and atmospheric humidity budget in arid inland river basins. However, knowledge of moisture recycling within many large inland basins and the factors that control moisture recycling is generally lacking. Based on a three-component isotopic mixing model, we assessed the characteristics of moisture recycling in China’s semi-arid Heihe River Basin. During the active growing season, almost half of the precipitation in the upper reaches was provided by local moisture recycling, and the main contribution came from transpiration. In the middle reaches, almost half of the precipitation in the artificial oasis and the desert-oasis ecotone was also provided by local moisture recycling, and the transpiration fraction (fTr) and evaporation fraction (fEv) of the artificial oasis differed from those of the desert-oasis ecotone. In the lower reaches, less than 25% of the precipitation was provided by local moisture recycling. Mean fTr values were relatively low in the Gobi (15.0%) in the middle reaches and in the riparian forest at Ejina (25.6%) in the lower reaches. The positive correlations between fTr and both precipitation and relative humidity suggest that higher precipitation and relative humidity promote transpiration fraction, whereas higher vapor pressure deficit reduces transpiration fraction. The positive correlation between fEv and temperature and vapor pressure deficit, and the negative correlation between fEv and relative humidity indicate that higher temperature and vapor pressure deficit promotes evaporation fraction, whereas higher relative humidity reduces the evaporation fraction. Our results show that contributions of recycled moisture (especially transpiration) to local precipitation play an important role in regional water resource redistribution in the arid and semi-arid region of northwestern China

Seasonality of the Transpiration Fraction and Its Controls Across Typical Ecosystems Within the Heihe River Basin

Understanding the seasonality of the transpiration fraction (T/ET) of total terrestrial evapotranspiration (ET) is vital for coupling ecological and hydrological systems and quantifying the heterogeneity among various ecosystems. In this study, a two‐source model was used to estimate T/ET in five ecosystems over the Heihe River Basin. In situ measurements of daily energy flux, sap flow, and surface soil temperature were compared with model outputs for 2014 and 2015. Agreement between model predictions and observations demonstrates good performance in capturing the ecosystem seasonality of T/ET. In addition, sensitivity analysis indicated that the model is insensitive to errors in measured input variables and parameters. T/ET among the five sites showed only slight interannual fluctuations while exhibited significant seasonality. All the ecosystems presented a single‐peak trend, reaching the maximum value in July and fluctuating day to day. During the growing season, average T/ET was the highest for the cropland ecosystem (0.80 ± 0.13), followed by the alpine meadow ecosystem (0.79 ± 0.12), the desert riparian forest Populus euphratica (0.67 ± 0.07), the Tamarix ramosissima Ledeb desert riparian shrub ecosystem (0.67 ± 0.06), and the alpine swamp meadow (0.55 ± 0.23). Leaf area index exerted a first‐order control on T/ET and showed divergence among the five ecosystems because of different vegetation dynamics and environmental conditions (e.g., water availability or vapor pressure deficits). This study quantified transpiration fraction across diverse ecosystems within the same water basin and emphasized the biotic controls on the seasonality of the transpiration fraction

Ecohydrology in water-limited environment using quantitative remote sensing - the Heihe River basin (China) case

Water-limited environments exist on all continents of the globe and they cover more than 30% of the Earth’s land surface. The eco-environments of these regions tend to be fragile and they are changing in a dramatic way through processes like land desertification, shrinking of oases, groundwater depletion, and soil erosion. These are either human induced or results of a changing climate. Implications of these changes for both the regional hydrologic cycle and the vegetation have been documented. Since these changes occur over a wide range of scales in space and time, remote sensing methods are needed to monitor the land surface characteristics, to observe changes in vegetation and hydrological states, and to compare these with predictions from hydrological models. It is widely accepted that remote sensing methods offer the ability to acquire spatially continuous measurements over large areas. Remote sensing can also help to visualize complex processes because the spatial data can be captured regularly over time. China is one of several countries with large arid and semi-arid areas. The Heihe River basin, situated in the arid inland of northwestern China, is one of the areas severely affected by ecoenvironmental degradation and recovery. The problem of the degraded environment is due to overexploitation of surface and ground water leading to shrinking of oases, including the decline and death of natural vegetation, and the lowering of the groundwater table. Exhaustive (over-)use of water resources is the main cause of land degradation in the lower reaches of the basin, called the Ejina oasis. The whole Heihe River basin is therefore selected as study area in this thesis to analyze the long-term eco-environmental changes. What happens in this river basin is likely to have a growing influence on regional hydrological cycles, even affecting human life. Effective management of eco-environmental problems in this critical zone of water-limited conditions will provide scientific evidence for protecting and improving the eco-environment in these Chinese northwestern arid regions, eventually resulting in land improvement. Studies on quantifying the relationship between the vegetation and the water resources are a critical step in developing an ecohydrological approach to resources management in order to minimize environmental degradation. Remote sensing measurements can help us to better understand the effects of changes in water management on hydrological processes and their subsequent feedback to the eco-environment at the regional scale. Remote sensing methods can also provide information to quantify heterogeneity and change at a large scale. Therefore, the main objective of this thesis is to develop a methodology for the quantitative assessment of eco-environmental changes at a large scale in arid regions by integrating remote sensing methods in ecohydrological approaches. Chapter 1 outlines the significance of quantitative assessment of eco-environmental changes using remote sensing methods and applying them for ecohydrology in northwestern China, resulting in the specific research objectives of this thesis. Chapter 2 quantifies both the vertical and horizontal distribution of vegetation in the Qilian Mountains area, representing the upper reaches of the Heihe River basin, based on MODIS NDVI images from the year 2000 - 2006. Our analysis reveals that elevation and aspect are two important impact factors for the vertical distribution of vegetation in a mountainous area. The NDVI increases with the elevation and reaches a maximum value at a certain elevation threshold, and then decreases as the elevation increases beyond this threshold. The optimal vegetation growth is on the shady side of the mountains because of less evapotranspiration. The best combination of temperature and precipitation is assessed providing good conditions for vegetation growth. Chapter 3 presents an efficient method to estimate the regional annual evapotranspiration (ET) based on the SEBS algorithm (Surface Energy Balance System) in the Zhangye basin, representing the middle reaches of the Heihe River basin. The method proposed is a combination of the daily SEBS results and data collected by meteorological stations. The result shows that the annual ET increased gradually during the period 1990-2004 and the main impact factor on the long-term increase of annual ET was the vegetation change. The accuracy of the ET result is validated using a water balance for the whole watershed and the validation reveals that the SEBS algorithm can be used to effectively estimate annual ET in the Zhangye basin. Chapter 4 establishes the quantitative relationship between the runoff of the Heihe River and the long-term vegetation change of the Ejina oasis, located in the lower reaches of the Heihe River. In this part, two time periods are distinguished corresponding to before and after the implementation of a new water allocation scheme in the Heihe River basin. The GIMMS NDVI and MODIS NDVI data sets are used to quantify the long-term change of the oasis vegetation in the first period 1989-2002 and the second period 2000-2006, respectively. The vegetation change shows a decreasing trend from 1989 to 2002 and an increasing trend between 2000 and 2006. Good relation between the runoff of the river and the vegetation growth are found at both stages and the time lag of the observed hysteresis effect of the runoff of the river on the oasis vegetation is one year. In addition, the yearly smallest water amount which sustains the demand of the eco-environment of the Ejina area is estimated to be 4×108 m3 based on MODIS images. Chapter 5 explores a method to quantify the effect of the groundwater depth on the vegetation growth in the year 2000 in the oasis area by combining MODIS NDVI with groundwater observation data. The result demonstrates that the groundwater depth suitable for vegetation growth in this region ranges from 2.8 to 5 m, depending on species composition. Hardly any vegetation growth occurs when the groundwater depth is below 5 m because the rooting depth of the occurring species is limited and cannot maintain adequate water supplies to their canopies when the water depth is below 5 m. The situation changes after implementation of the new water allocation scheme since 2000. The mean NDVI increased and the annual conversion of bare land into vegetated land is about 38 km2 per year during the period 2000 – 2008. It reflects a potential recovery of the eco-environment of the Ejina area. Chapter 6 comprises the main conclusions and the outlook for possible improvements in future research. The main contribution of this study is the successful integration of remote sensing with ecohydrology in quantifying the relationship between water resources and vegetation occurrence at large scale. It provides a methodology to evaluate the long-term vegetation change and the water resources impact using remote sensing data in water-limited areas. The approach of vegetation dynamics, runoff and groundwater impacts presented in this thesis serves as a sound foundation for predicting the effects of future environmental changes. <br/

Attribution of hydrological change in Heihe River Basin to climate and land use change in the past three decades

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Accounting of public preferences and valuation of terrestrial and aquatic ecosystem services restoration: Evidence from Northwestern China

The emphasis of current study is on the assessment of households’ willingness to pay (WTP) and preference heterogeneity for the upgradation of degraded terrestrial and aquatic ecosystem services across sub-basins of Heihe River. A sum of 1679 households were interviewed through choice experiment, and random parameter logit model is applied for welfare estimation. The estimated results signified the importance of selected terrestrial and aquatic ecological attributes. Sampled respondents of upper and middle basins placed highest WTP for reducing the emission of greenhouse gases among all other attributes and the WTP amount was 99.27 and 66.49 Yuan/Year respectively. Whereas in lower basin, improvement in landscape of farmland was highly valued, i.e., 107 Yuan/Year. In terrestrial ecosystem services the lowest WTP was recorded for Ejinaqi oasis size in all sub basins. In aquatic ecosystem services, improving water quality was the most valued attribute in all sub basins, and the corresponding WTP values are 122.81, 99.87 and 177.43 Yuan/Year in upper, middle and lower basins respectively, which ensure the significance of water quality in the entire basin. In aquatic ecosystem services the lowest WTP amount was recorded for leisure and entertainment conditions in each sub basin.publishedVersio

Convection-permitting fully coupled WRF-Hydro ensemble simulations in high mountain environment: impact of boundary layer- and lateral flow parameterizations on land–atmosphere interactions

Numerical climate models have been upgraded by the improved description of terrestrial hydrological processes across different scales. The goal of this study is to explore the role of terrestrial hydrological processes on land–atmosphere interactions within the context of modeling uncertainties related to model physics parameterization. The models applied are the Weather Research and Forecasting (WRF) model and its coupled hydrological modeling system WRF-Hydro, which depicts the lateral terrestrial hydrological processes and further allows their feedback to the atmosphere. We conducted convection-permitting simulations (3 km) over the Heihe River Basin in Northwest China for the period 2008–2010, and particularly focused on its upper reach area of complex high mountains. In order to account for the modeling uncertainties associated with model physics parameterization, an ensemble of simulations is generated by varying the planetary boundary layer (PBL) schemes. We embedded the fully three-dimensional atmospheric water tagging method in both WRF and WRF-Hydro for quantifying the strength of land–atmosphere interactions. The impact of PBL parameterization on land–atmosphere interactions is evaluated through its direct effect on vertical mixing. Results suggest that enabled lateral terrestrial flow in WRF-Hydro distinctly increases soil moisture and evapotranspiration near the surface in the high mountains, thereby modifies the atmospheric condition regardless of the applied PBL scheme. The local precipitation recycling ratio in the study area increases from 1.52 to 1.9% due to the description of lateral terrestrial flow, and such positive feedback processes are irrespective of the modeling variability caused by PBL parameterizations. This study highlights the non-negligible contribution of lateral terrestrial flow to local precipitation recycling, indicating the potential of the fully coupled modeling in land–atmosphere interactions research