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Spatial distribution of reference evapotranspiration (ET0) is essential in water resources planning and management, especially in semi-arid areas. In this paper, a digital elevation model is used in an `interpolate-then-calculate' approach to calculating the spatially distributed ET0 using the physically based Penman-Monteith equation in the Taoer river basin in China. The results show the following. (1) Of 11 interpolation methods, the Inverse Distance Weighting (IDW) method was found to be best for interpolating wind speed and a tn-variate secondary trend surface method was found most suitable for interpolating mean air temperature and relative humidity. Spatial modelling of the radiation environment considered the effects of elevation, slope and aspect. (2) Monthly values in January for the three meteorological variables showed larger spatial variations than in July, and just the reverse of net surface radiation. (3) The resulting ET0 calculated at each grid cell with 200m resolution and its spatial variation showed strong seasonal variation. Lower ET0 was found in high-elevation southern Great Xingan mountains in the northwest basin, while higher values were located in the plains adjacent to the lower reach. (4) The ET0 distribution by the `interpolate-then-calculate' approach better reflected the effects of topography than that of the `calculate-then-interpolate' approach. [ABSTRACT FROM AUTHOR]

Abstract: The nonparametric Mann–Kendall test was used to detect the trends of major hydroclimatic variables in the Tarim River Basin, the largest inland river basin in China for the period of 1960–2007. Results showed that both mean annual air temperature and precipitation experienced an increasing trend, while annual streamflow demonstrated a mixed trend of decreasing and increasing: The mountainous region upstream showed an increasing trend and the region downstream exhibited a decreasing trend. Impacts of the increased air temperature on streamflow have shown different characteristics depending on location and seasons: it has positive effect on the runoff at mountainous region due to snowmelt and glacier-melt in spring, but negative effect on the runoff at plain area due to the increase of actual evaporation in summer. In addition, human activity contributed to the declining of streamflow in the arid plain oases at downstream of the Tarim River Basin. The results obtained in this paper can be used as a reference for the planning and management of water resources to maintain the health of the river system. [Copyright &y& Elsevier]

Information on evapotranspiration (ET) can help us understand water balance, particularly in forested watersheds. Previous studies in China show that ET was relatively low (30-40% of total precipitation) in the Minjiang Valley located in the upper reach of the Yangtze River Basin. However, this conclusion was derived from research on small-scale watersheds (<100 km2). The objective of this paper was to present ET information on meso-scale watersheds in the Minjiang Valley. Four meso-scale watersheds (1,700-5,600 km2) located in the Minjiang Valley were used to estimate ET using the water balance approach. We first generated forest vegetation variables (coniferous forest percentage, forest cover percentage, and derived forest vegetation index) using remote sensing data. Landsat 5 TM satellite images, acquired on June 26, 1994, were selected for the vegetation classification. Actual annual ET was calculated based on 11-year estimated precipitation and measured streamflow data (1992-2002). We also calculated potential ET (PET) using an improved Thornthwaite model for all four watersheds for the period of 1992-1998. PET can provide additional information about potential capacity of water flux to atmosphere in the region. Seasonal (dry and rainy) PET and ET for all studied watersheds were also estimated for comparison purposes as the water balance approach, at shorter than annual scales, would likely provide inaccurate estimates of ET. The dominant vegetations in the Minjiang Valley were grasslands, conifer forests, and shrub-lands. Our results confirmed that both ET and PET for three studied meso-scale watersheds in the Minjiang Valley is relatively low (39.5-43.8 and 28.2-47.7% for ET and PET, respectively), with an exception of ET in the Yuzixi watershed being 71.1%. This result is generally consistent with previous research at small watershed scales. Furthermore, the low ET across various scales in the Minjiang Valley may be related to the unique deeply cut valley environment. [ABSTRACT FROM AUTHOR]

Natural forests in southern China have been severely logged due to high human demand for timber, food, and fuels during the past century, but are recovering in the past decade. The objective of this study was to investigate how vegetation cover changes in composition and structure affected the water budgets of a 9.6-km2 Dakeng watershed located in a humid subtropical mountainous region in southern China. We analyzed 27 years (i.e., 1967-1993) of streamflow and climate data and associated vegetation cover change in the watershed. Land use/land cover census and Normalized Difference of Vegetation Index (NDVI) data derived from remote sensing were used to construct historic land cover change patterns. We found that over the period of record, annual streamflow ( Q) and runoff/precipitation ratio did not change significantly, nor did the climatic variables, including air temperature, Hamon’s potential evapotranspiration (ET), pan evaporation, sunshine hours, and radiation. However, annual ET estimated as the differences between P and Q showed a statistically significant increasing trend. Overall, the NDVI of the watershed had a significant increasing trend in the peak spring growing season. This study concluded that watershed ecosystem ET increased as the vegetation cover shifted from low stock forests to shrub and grasslands that had higher ET rates. A conceptual model was developed for the study watershed to describe the vegetation cover-streamflow relationships during a 50-year time frame. This paper highlighted the importance of eco-physiologically based studies in understanding transitory, nonstationary effects of deforestation or forestation on watershed water balances. [ABSTRACT FROM AUTHOR]