Climate, canopy conductance and leaf area development controls on evapotranspiration in a boreal coniferous forest over a 10-year period: A united model assessment

The aim of this work was to test a process-based model (hydrological model combined with forest growth model) on the simulation of seasonal variability of evapotranspiration (ET) in an even-aged boreal Scots pine (Pinus sylvestris L.) stand over a 10 year period (1999-2008). The water flux components (including canopy transpiration (E_t) and evaporation from canopy (E_c) and ground surface (E_g) were estimated in order to output the long-term stand water budget considering the interaction between climate variations and stand development. For validation, half-hourly data on eddy water vapor fluxes were measured during the 10 growing seasons (May-September). The model predicted well the seasonal course of ET compared to the measured values, but slightly underestimated the water fluxes both in non-drought and drought (2000, 2003 and 2006) years. The prediction accuracy was, on average, higher in drought years. The simulated ET over the 10 years explained, on average, 58% of the daily variations and 84% of the monthly amount of ET. Water amount from E_t contributed most to the ET, with the fractions of E_t, E_c and E_g being, on average, 67, 11 and 23% over the 10-year period, respectively. Regardless of weather conditions, the daily ET was strongly dependent on air temperature (T_a) and vapor pressure deficit (D_a), but less dependent on soil moisture (W_s). On cloudy and rainy days, there was a non-linear relationship between the ET and solar radiation (R_o). During drought years, the model predicted lower daily canopy stomatal conductance (g_cs) compared with non-drought years, leading to a lower level of E_t. The modeled daily g_cs responded well to D_a and W_s. In the model simulation, the annual LAI increased by 35% between 1999 and 2008. The ratio of E_c: ET correlated strongly with LAI. Furthermore, LAI reduced the proportion of E_g as a result of the increased share of E_c and E_t and radiation interception. Although the increase of LAI affected positively E_t, the contribution of E_t in ET was not significantly correlated with LAI. To conclude, although the model predicted reasonably well the seasonal course of ET, the calculation time steps of different processes in the model should be homogenized in the future to increase the prediction accuracy.     

甘肃河东玉米种植区春夏气象干旱时空变化特征及其与环流因子关系

甘肃河东玉米种植区属于旱作雨养农业区,农作物生长主要依靠自然降水来维持,春夏干旱是影响该区玉米生长发育的关键因素。利用河东地区13个气象站点1957-2012年气象资料,基于标准化降水蒸散指数从干旱趋势、周期、空间特征及其与环流因子关系方面探讨近56 a来河东玉米种植区春夏干旱演变特征。结果表明：近56 a河东玉米种植区春夏两季均呈干旱化趋势,并自20世纪90年代中期以后,干旱呈加重趋势。干旱变化共经历了3个气候阶段,20世纪80年代之前,春夏旱情较轻;80年代-90年代,夏旱较春旱严重;2000年以后,春旱较夏旱严重。干旱周期变化,春夏干旱周期分别为20 a和14 a,干旱周期性变化主要是受太阳黑子活动影响。春夏干旱发生频率在突变前后差异显著,低频区在突变后转为高频区,易旱区呈扩大状态。ENSO事件是影响该区春夏干旱发生的主要环流因子,尤其对夏旱影响最为显著。近20 a来ENSO事件增强是该区干旱事件不断加重的原因,ENSO暖事件爆发时,该区春夏趋于干旱,由湿向干转变;ENSO冷事件爆发,干旱则有所缓解,由干向湿转变。     

Using Landsat data to estimate evapotranspiration of winter wheat

An evapotranspiration (ET) model that accurately estimates daily water use and soil moisture on a regional basis is required for many agricultural and hydrological studies. The model should use meterological data that are readily available and crop information that is responsive to the changing vigor of the plants.We evaluated an ET model with a weighing lysimeter and then applied it to winter wheatfields at four Kansas locations. Model inputs are solar radiation, temperature, precipitation, and leaf area index (LAI); included in the outputs are estimates of transpiration, evaporation, and soil moisture. An equation was developed to estimate LAI from Landsat data. Because LAI can be estimated from satellites, the ET model can potentially be used on a regional basis.     

Control of Tamarix in the Western United States: Implications for Water Salvage, Wildlife Use, and Riparian Restoration

Non-native shrub species in the genus Tamarix (saltcedar, tamarisk) have colonized hundreds of thousands of hectares of floodplains, reservoir margins, and other wetlands in western North America. Many resource managers seek to reduce saltcedar abundance and control its spread to increase the flow of water in streams that might otherwise be lost to evapotranspiration, to restore native riparian (streamside) vegetation, and to improve wildlife habitat. However, increased water yield might not always occur and has been substantially lower than expected in water salvage experiments, the potential for successful revegetation is variable, and not all wildlife taxa clearly prefer native plant habitats over saltcedar. As a result, there is considerable debate surrounding saltcedar control efforts. We review the literature on saltcedar control, water use, wildlife use, and riparian restoration to provide resource managers, researchers, and policy-makers with a balanced summary of the state of the science. To best ensure that the desired outcomes of removal programs are met, scientists and resource managers should use existing information and methodologies to carefully select and prioritize sites for removal, apply the most appropriate and cost-effective control methods, and then rigorously monitor control efficacy, revegetation success, water yield changes, and wildlife use.     

Cost/Benefit Considerations for Recent Saltcedar Control,Middle Pecos River,New Mexico

Major benefits were weighed against major costs associated with recent saltcedar control efforts along the Middle Pecos River, New Mexico. The area of study was restricted to both sides of the channel and excluded tributaries along the 370 km between Sumner and Brantley dams. Direct costs (helicopter spraying, dead tree removal, and revegetation) within the study area were estimated to be $2.2 million but possibly rising to $6.4 million with the adoption of an aggressive revegetation program. Indirect costs associated with increased potential for erosion and reservoir sedimentation would raise the costs due to increased evaporation from more extensive shallows in the Pecos River as it enters Brantley Reservoir. Actions such as dredging are unlikely given the conservative amount of sediment calculated (about 1% of the reservoir pool). The potential for water salvage was identified as the only tangible benefit likely to be realized under the current control strategy. Estimates of evapotranspiration (ET) using Landsat TM data allowed estimation of potential water salvage as the difference in ET before and after treatment, an amount totaling 7.41 million m³ (6010 acre-ft) per year. Previous saltcedar control efforts of roughly the same magnitude found that salvaged ET recharged groundwater and no additional flows were realized within the river. Thus, the value of this recharge is probably less than the lowest value quoted for actual in-channel flow, and estimated to be < $63,000 per year. Though couched in terms of costs and benefits, this paper is focused on what can be considered the key trade-off under a complete eradication strategy: water salvage vs. erosion and sedimentation. It differs from previous efforts by focusing on evaluating the impacts of actual control efforts within a specific system. Total costs (direct plus potential indirect) far outweighed benefits in this simple comparison and are expected to be ongoing. Problems induced by saltcedar control may permanently reduce reservoir capacity and increase reservoir evaporation rates, which could further deplete supplies on this water short system. These potential negative consequences highlight that such costs and benefits need to be considered before initiating extensive saltcedar control programs on river systems of the western United States.     

Hydroclimatic changes and drivers in the Sava River Catchment and comparison with Swedish catchments

In this study, we investigate long-term hydroclimatic changes and their possible relation to regional changes in climate, land-use and water-use over the twentieth century in the transboundary Sava River Catchment (SRC) in South Eastern Europe. In a hydropower dominated part of the SRC, unlike in an unregulated part, we find increase in average annual evapotranspiration and decrease in temporal runoff variability, which are not readily explainable by observed concurrent climate change in temperature and precipitation and may be more related to landscape-internal change drivers. Among the latter investigated here, results indicate hydropower developments as most closely related to the found hydroclimatic shifts, consistent with previous such indications in studies of Swedish hydropower catchments. Overall, the present results have quantitatively framed the recent history and present state of hydroclimate in the SRC, of relevance for water resources in several countries and for a majority of their populations. This provides a useful basis for further assessment of possible future hydroclimatic changes, under different scenarios of climate change and land/water-use developments in the region.

Electronic supplementary material

The online version of this article (doi:10.1007/s13280-015-0641-0) contains supplementary material, which is available to authorized users.     

基于SPEI的西南地区近53 a干旱时空特征分析

论文基于标准化降水蒸散指数（SPEI）,统计分析西南地区128 个测站1960-2012 年的气象数据,从干旱年际变化趋势、四季变化趋势、干旱强度、干旱事件频次、干旱频率以及与ENSO的关系,对西南地区近半个世纪的干旱时空特征进行了分析。结果表明：①西南地区及子区域近53 a 来呈干旱化趋势,21 世纪初干旱发生最频繁,干旱强度、极端干旱及中等干旱的频次均呈增加趋势;②四季大部分区域呈干旱化趋势,以秋季最为突出;③春季,干旱发生频率最高且集中在横断山地、四川盆地东部和云贵高原中部,夏季,横断山地北部、若尔盖高原和广西丘陵西北部易发生干旱,秋季,云贵高原、广西丘陵及四川盆地部分区域干旱频率较高,冬季,干旱易发区集中在若尔盖高原、四川盆地西南部一线;④各区域四季的干旱指数与ENSO指数相关性不同,并且ENSO事件强度与四川盆地和横断山地的SPEI 在年变化趋势方面存在明显负相关,与其他区域呈正相关。此外,西南地区在厄尔尼诺年和拉尼娜年都会出现干旱,但前者爆发干旱灾害的概率比后者高。而且各区域存在差异,四川盆地、若尔盖高原在厄尔尼诺年易发生干旱,而云贵高原在拉尼娜年发生频率较高,广西丘陵、横断山地没有明显规律和特征。