Abstract:Evapotranspiration is an important component of the hydrological cycle, which integrates atmospheric demands and surface conditions. Research on spatial and temporal variations of reference evapotranspiration (ET o ) enables understanding of climate change and its effects on hydrological processes and water resources. In this study, ET o was estimated by the FAO-56 PenmanMonteith method in the Jing River Basin in China, based on daily data from 37 meteorological stations from 1960 to 2005. ET o trends were detected by the Mann-Kendall test in annual, seasonal, and monthly timescales. Sensitivity coefficients were used to examine the contribution of important meteorological variables to ET o . The influence of agricultural activities, especially irrigation on ET o was also analyzed. We found that ET o showed a decreasing trend in most of the basin in all seasons, except for autumn, which showed an increasing trend. Mean maximum temperature was generally the most sensitive parameter for ET o , followed by relative humidity, solar radiation, mean minimum temperature, and wind speed. Wind speed was the most dominant factor for the declining trend in ET o . The more significant decrease in ET o for agricultural and irrigation stations was mainly because of the more significant decrease in wind speed and sunshine hours, a mitigation in climate warming, and more significant increase in relative humidity compared with natural stations and non-irrigation stations. Changes in ET o and the sensitivity coefficient of meteorological variables in relation to ET o were also affected by topography. Better understanding of ET o response to climate change will enable efficient use of agricultural production and water resources, which could improve the ecological environment in Jing River Basin.

Large-scale forestation has been undertaken over decades principally to control the serious soil erosion in the Loess Plateau of China. A quantitative assessment of the hydrological effects of forestation, especially on basin water yield, is critical for the sustainable forestry development within this dry region. In this study, we constructed the multi-annual water balances to estimate the respective grand average of annual evapotranspiration (ET) and runoff for forestlands and non-forestlands of 57 basins. The overall annual runoff and corresponding runoff/precipitation ratio were low, with a mean of 33 mm (7%) ranging from 10 (2%) to 56 mm (15%). Taking the grand average of annual precipitation of 463 mm for all basins, the corresponding grand averages of annual ET and runoff were 447 and 16 mm for forestlands, 424 and 39 mm for non-forestlands, respectively. Thus, the corresponding ratios of annual ET and runoff to precipitation were 91Ð7 and 8Ð3% for non-forestlands, 96Ð6 and 3Ð4% for forestlands, respectively. Although the absolute difference in grand average of annual runoff was only 23 mm, it represents a large difference in relative terms, as it equates up to 58% of annual runoff from non-forestlands. We argue that the large-scale forestation may have serious consequences for water management and sustainable development in the dry region of NW China because of a runoff reduction. This study highlights the importance of quantifying the ET of forests and other land uses and to examine how land cover change may affect the water balances in an arid environment.

The increase of coverage of forest ⁄ vegetation is imperative to improve the environment in dry-land areas of China, especially for protecting soil against serious erosion and sandstorms. However, inherent severe water shortages, drought stresses, and increasing water use competition greatly restrict the reforestation. Notably, the water-yield reduction after afforestation generates intense debate about the correct approach to afforestation and forest management in dry-land areas. However, most studies on water-yield reduction of forests have been at catchment scales, and there are few studies of the response of total evapotranspiration (ET) and its partitioning to vegetation structure change. This motivates us to learn the linkage between hydrological processes and vegetation structure in slope ecosystems. Therefore, an ecohydrological study was carried out by measuring the individual items of water balance on sloping plots covered by different vegetation types in the semiarid Liupan Mountains of northwest China. The ratio of precipitation consumed as ET was about 60% for grassland, 93% for shrubs, and >95% for forestland. Thus, the water yield was very low, site-specific, and sensitive to vegetation change. Conversion of grassland to forest decreased the annual water yield from slope by 50-100 mm. In certain periods, the plantations at lower slopes even consumed the runon from upper slopes. Reducing the density of forests and shrubs by thinning was not an efficient approach to minimize water use. Leaf area index was a better indicator than plant density to relate ET to vegetation structure and to evaluate the soil water carrying capacity for vegetation (i.e., the maximum amount of vegetation that can be supported by the available soil water for an extended time). Selecting proper vegetation types and plant species, based on site soil water condition, may be more effective than the forest density regulation to minimize water-yield reduction by vegetation coverage increase and notably by reforestation. Finally, the focuses in future research to improve the forest-water relations in dry-land areas are recommended as follows: vegetation growth dynamics driven by environment especially water conditions, coupling of ecological and hydrological processes, further development of distributed ecohydrological models, quantitative relation of ecowater quota of ecosystems with vegetation structures, multi-scaled evaluation of soil water carrying capacity for vegetation, and the development of widely applicable decision support tools.(KEY TERMS: forests; land use ⁄ land cover change; watershed management; evapotranspiration; sap flow; water yield; dry-land area; China.)

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An accurate prediction of forest evapotranspiration (ET) based on its components in response to a changing environment is essential for understanding interactions between the atmosphere, soil, and vegetation and for integrated forest‐water management. The ET components of a pure larch plantation and the environment were monitored over 2 years in northwest China. The response functions of each ET component to individual driving factors were determined using upper boundary lines, then coupled to form the ET component modules, fitted with measured data in 2016 (May–September), and validated with measured data in 2015 (June–September). Results showed that (1) the response of daily transpiration (T) to potential ET (ETref) followed a binomial equation, and the response of T to relative extractable water (REW) of the 0‐ to 60‐cm soil layer and canopy leaf area index (LAI) followed a saturated exponential growth function. The module was T = (−5.766 × 10−4ETref2 + 0.005ETref–0.002) × (18.769 + 46.990 (1–exp(−8.555REW))) × (−14.662 + 17.428 (1–exp(−1.414LAI))). (2) The response of daily forest floor evapotranspiration (FE) to ETref, volumetric soil moisture (VSM) of the 0‐ to 30‐cm soil layer, and LAI followed positive linear, saturated exponential growth and saturated exponential decay function. The module was FE = (6.697ETref–2.770) × (6.927–11.243exp(−1.959VSM)) × (0.032 + 1.162exp(−2.407LAI)). (3) The canopy interception (Ic) of individual rainfall events was mainly affected by precipitation amount (P) and LAI. The module was Ic = 0.446 LAI (1–exp(−0.112P)) + 0.097P. (4) The daily ET model was built up as ET = T + FE + Ic, which had good performance in both the calibration and validation period. It can be concluded that the variation of daily forest ET can be accurately predicted using the easily measurable driving factors of weather, soil, and vegetation.

Mountainous forest areas are vitally important for water supply in dryland regions which suffer from high erosion risk and severe water shortage. Massive afforestation, mainly for erosion control, may reduce the water yield and threaten local water supply security. Moreover, many over-dense forests due to a strict logging ban policy have produced remarkably negative impacts for both forests (e.g., low timber quality, restricted natural regeneration, and high stand instability) and water yield. To satisfy the rapidly increasing demands on water supply and other services, a practical approach for managing forest stands in a multifunctional way, which particularly addresses water yielding, is urgently required. For this purpose, we integrated the existing knowledge and experience, designed an "ideal" stand structure to represent multifunctional forest (MFF) and determined its key parameters (a ground coverage of >0.7, a canopy density around 0.7, and an H/DBH ratio (tree height [m] to the diameter at breast height [cm]) of <0.7). Moreover, a decision process for MFF stand management was recommended as: (1) investigating the site quality; (2) identifying the site-specific main forest functions; (3) quantifying the stand structure; (4) diagnosing the stand structure by comparing with the "ideal" one; and (5) arranging the functions/structure-oriented management measures. In this way, the water-yielding function can be improved and meanwhile other forest functions can be promoted.(KEY TERMS: dryland regions; forest hydrology; mountain forests; multifunctionality; best management practices; water allocation; water supply; runoff.)

Small differences in the sensitivity of stomatal conductance to light intensity on leaf surfaces may lead to large differences in total canopy transpiration (EC) with increasing canopy leaf area (L). Typically, the increase of L would more than compensate for the decrease of transpiration per unit of leaf area (EL), resulting in concurrent increase of EC. However, highly shade-intolerant species, such as Larix principis-rupprechtii Mayr., may be so sensitive to increased shading that such compensation is not complete. We hypothesized that in such a stand, windfall-induced spatial variation at a decameter scale would result in greatly reduced EL in patches of high L leading to lower EC than low competition patches of sparse canopy. We further hypothesized that quicker extraction of soil moisture in patches of lower competition will result in earlier onset of drought symptoms in these patches. Thus, patches of low L will transition from light to soil moisture as the factor dominating EL. This process should progressively homogenize EC in the stand even as the variation of soil moisture is increasing. We tested the hypotheses utilizing sap flux of nine trees, and associated environmental and stand variables. The results were consistent with only some of the expectations. Under non-limiting soil moisture, EL was very sensitive to the spatial variation of L, decreasing sharply with increasing L and associated decrease of mean light intensity on leaf surfaces. Thus, under the conditions of ample soil moisture maximum EC decreased with increasing patch-scale L. Annual EC and biomass production also decreased with L, albeit more weakly. Furthermore, variation of EC among patches decreased as average stand soil moisture declined between rain events. However, contrary to expectation, high L plots which transpired less showed a greater EL sensitivity to decreasing stand-scale soil moisture, suggesting a different mechanism than simple control by decreasing soil moisture. We offer potential explanations to the observed phenomenon. Our results demonstrate that spatial variation of L at decameter scale, even within relatively homogeneous, single-species, even-aged stands, can produce large variation of transpiration, soil moisture and biomass production and should be considered in 1-D soil-plant-atmosphere models.

[1] The eco-hydrological model SWIM was used to examine the effects of forestation on water yield in a watershed of the Liupan Mountains in northwest China. The results showed that the water yield variation caused by tree species shift among mature forests dominated by larch, poplar and birch was negligible. The vegetation type conversion from grassland to forest strongly reduced water yield. The annual water yield reduction after 10% forestation was 15.8 mm on average with a fluctuation from 3.5 to 19.3 mm. The contribution of site variation to water yield varied from a decrease of 3.5 mm to an increase of 12.3 mm after 10% forestation, which on average was nearly a half of the influence of vegetation conversion between forest and grassland. Site selection for forestation in mountainous areas could be beneficial in alleviating forest-water conflicts and lessening the water yield reduction caused by forestation.