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1.
Conservation management for the water dependent desert‐oasis ecotone in arid northwest China requires information on the water use of the dominant species. However, no studies have quantified their combined water use or linked species composition to ecotone transpiration. Here, the water use of three dominant shelterbelt shrubs (Haloxylon ammodendron, Nitraria tangutorum, and Calligonum mongolicum) within an ecotone was measured throughout the full leaf‐out period for three shrub species from 30 May to 16 October 2014, with sap flow gauges using the stem heat balance approach. Species‐specific transpiration was estimated by scaling up sap flow velocities measured in individual stems, to stand area level, using the frequency distribution of stem diameter and assuming a constant proportionality between sap flow velocity and basal cross‐sectional area for all stems. The mean peak sap flux densities (Jsn) for H. ammodendron, N. tangutorum, and C. mongolicum, were 40.12 g cm?2 h?1, 71.33 g cm?2 h?1, and 60.34 g cm?2 h?1, respectively, and the mean estimated daily area‐averaged transpiration rates (Tdaily) for the same species were 0.56 mm day?1, 0.34 mm day?1, and 0.11 mm day?1. The accumulative stand transpiration was approximately 140.8 mm throughout the measurement period, exceeding precipitation by as much as 42.1 mm. Furthermore, Tdaily of these shrubs appeared to be much less sensitive to soil moisture as compared to atmospheric drivers, and the relationship between Jsn and atmospheric drivers was likely uninfluenced by soil moisture regimes in the whole profile (to 1‐m depth), especially for H. ammodendron and C. mongolicum. Results indicate that these shrubs may use deep soil water recharged by capillary rise, or may directly access shallow groundwater. This study provides quantitative data offering important implications for ecotone conservation and water and land resource management. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

2.
Estimation of evapotranspiration (ET) is of great significance in modeling the water and energy interactions between land and atmosphere. Negative correlation of surface temperature (Ts) versus vegetation index (VI) from remote sensing data provides diagnosis on the spatial pattern of surface soil moisture and ET. This study further examined the applicability of Ts–VI triangle method with a newly developed edges determination technique in estimating regional evaporative fraction (EF) and ET at MODIS pixel scale through comparison with large aperture scintillometer (LAS) and high‐level eddy covariance measurements collected at Changwu agro‐ecological experiment station from late June to late October, 2009. An algorithm with merely land and atmosphere products from MODIS onboard Terra satellite was used to estimate the surface net radiation (Rn) and soil heat flux. In most cases, the estimated instantaneous Rn was in good agreement with surface measurement with slight overestimation by 12 W/m2. Validation results from LAS measurement showed that the root mean square error is 0.097 for instantaneous EF, 48 W/m2 for instantaneous sensible heat flux, and 30 W/m2 for daily latent heat flux. This paper successfully presents a miniature of the overall capability of Ts–VI triangle in estimating regional EF and ET from limited number of data. For a thorough interpretation, further comprehensive investigation needs to be done with more integration of remote sensing data and in‐situ surface measurements. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

3.
Evapotranspiration (ET) from riparian vegetation can be difficult to estimate due to relatively abundant water supply, spatial vegetation heterogeneity, and interactions with anthropogenic influences such as shallower groundwater tables, increased salinity, and nonpoint source pollution induced by irrigation. In semiarid south-eastern Colorado, reliable ET estimates are scarce for the riparian corridor that borders the Arkansas River. This work investigates relationships between the riparian ecosystem along the Arkansas River and an underlying alluvial aquifer using ET estimates from remotely sensed data and modelled water table depths. Results from a calibrated, finite-difference groundwater model are used to estimate weekly water table fluctuations in the riparian ecosystem from 1999 to 2009, and estimates of ET are calculated using the Operational Simplified Surface Energy Balance (SSEBop) model with over 200 Landsat scenes covering over 30 km2 of riparian ecosystem along a 70-km stretch of the river. Comparison of calculated monthly SSEBop ET to estimated alfalfa reference ET from local micrometeorological station data indicated statistically significant high linear correspondence (R2 = .87). Daily calculated SSEBop ET showed statistically significant moderate linear correspondence with data from a local weighing lysimeter (R2 = .59). Simulated monthly SSEBop ET values were larger in drier years compared with wetter years, and ET variability was also larger in drier years. Peak ET most commonly occurred during the month of June for all 11 years of analysis. Relationships between ET and water table depth showed that peak monthly ET was highest when groundwater depths were less than about 3 m, and ET values were significantly lower for groundwater depths greater than 3 m. Negative sample Spearman correlation highlighted riparian corridor locations where ET increased as a result of decreased groundwater depths across years with different hydroclimatic conditions. This study shows how a combination of remotely sensed riparian ET estimates and a regional groundwater model can improve our understanding of linkages between riparian consumptive use and near-river groundwater conditions influenced by irrigation return flow and different climatic drivers.  相似文献   

4.
Abstract

Reliable estimation of sensible heat flux (H) is important in energy balance models for quantifying evapotranspiration (ET). This study was conducted to evaluate the value of adding the Priestley-Taylor (PT) equation to the METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) model. METRIC was used to estimate energy fluxes for 10 Landsat images from the 2005, 2006 and 2007 crop growing seasons in south-central Nebraska, USA, where each image owing to recent rainfall exhibited high residual moisture content even at the hot pixel. The METRIC model performed satisfactorily for net radiation (Rn ) and soil heat flux (G) estimation with a root mean square error (RMSE) of 52 and 24 W m-2, respectively. A RMSE of 122 W m-2 for H indicated the limitation of the METRIC model in estimating H for high residual moisture content of the hot pixel (Alfalfa reference ET fraction, ET r F > 0.15). The modified METRIC model (wet METRIC or wMETRIC) incorporating the PT equation was applied to calculate H at the anchor pixels (hot and cold) for high residual moisture content of the hot pixel. The α coefficient of the PT equation was locally calibrated using hourly meteorological data from an automatic weather station and Rn and G data from a Bowen ratio flux tower. The mean α coefficient value was 1.14. The wMETRIC model reduced the RMSE of H from 122 to 64 W m-2 and that of latent heat flux, LE, from 163 to 106 W m-2. The RMSE of daily ET decreased from 1.7 to 1.1 mm d-1 with wMETRIC. The results indicate that treatment of anchor pixels for high residual moisture content with the PT approach gives improved estimation of H, LE and daily ET. It is recommended that the wMETRIC model be used for estimating ET if the hot pixel has high residual moisture (i.e. reference ET fraction > 0.15).

Citation Singh, R. K. & Irmak, A. (2011) Treatment of anchor pixels in the METRIC model for improved estimation of sensible and latent heat fluxes. Hydrol. Sci. J. 56(5), 895–906.  相似文献   

5.
Because groundwater recharge in dry regions is generally low, arid and semiarid environments have been considered well-suited for long-term isolation of hazardous materials (e.g., radioactive waste). In these dry regions, water lost (transpired) by plants and evaporated from the soil surface, collectively termed evapotranspiration (ET), is usually the primary discharge component in the water balance. Therefore, vegetation can potentially affect groundwater flow and contaminant transport at waste disposal sites. We studied vegetation health and ET dynamics at a Uranium Mill Tailings Radiation Control Act (UMTRCA) disposal site in Shiprock, New Mexico, where a floodplain alluvial aquifer was contaminated by mill effluent. Vegetation on the floodplain was predominantly deep-rooted, non-native tamarisk shrubs (Tamarix sp.). After the introduction of the tamarisk beetle (Diorhabda sp.) as a biocontrol agent, the health of the invasive tamarisk on the Shiprock floodplain declined. We used Landsat normalized difference vegetation index (NDVI) data to measure greenness and a remote sensing algorithm to estimate landscape-scale ET along the floodplain of the UMTRCA site in Shiprock prior to (2000–2009) and after (2010–2018) beetle establishment. Using groundwater level data collected from 2011 to 2014, we also assessed the role of ET in explaining seasonal variations in depth to water of the floodplain. Growing season scaled NDVI decreased 30% (p < .001), while ET decreased 26% from the pre- to post-beetle period and seasonal ET estimates were significantly correlated with groundwater levels from 2011 to 2014 (r2 = .71; p = .009). Tamarisk greenness (a proxy for health) was significantly affected by Diorhabda but has partially recovered since 2012. Despite this, increased ET demand in the summer/fall period might reduce contaminant transport to the San Juan River during this period.  相似文献   

6.
Daily evapotranspiration from a winter wheat field on the North China Plain measured by large‐scale weighing lysimeter was linearly related to that measured by the Bowen ratio energy balance (BREB) technique. Soil evaporation averaged about 23·6% of evapotranspiration from the post‐winter dormancy revival stage to the grain ripening stage in 1999. On clear days during winter dormancy, about half of the net radiation flux Rn was used to warm soil. During the revival stage, conductive heat flux G also used most of the incoming Rn, but the ratio of latent heat flux λE to Rn increased. During the stem‐extension stage, λE was about 50% of Rn; thereafter, λE/Rn increased continually, but G remained less than 10% of Rn. During the ripening stage, λE was almost 90% of Rn. Evaporative fraction (EF) can be expressed as a function of plant status and atmospheric boundary layer conditions. The relationship between EF and available energy under moderate air temperature and vapour pressure deficit conditions was examined for five combinations of aerodynamic and canopy conductance. Although the theoretical relationship indicates that EF should be highly correlated to soil water content, the correlation has been difficult to identify under field conditions. However, we observed that there exists a threshold value of Rn ? G, above which EF is less than 1·0, and that the threshold value is lower under soil‐water deficit conditions than under abundant soil‐water conditions. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

7.
High groundwater salinity has become a major concern in the arid alluvial plain of the Dunhuang Basin in northwestern China because it poses a significant challenge to water resource management. Isotopic and geochemical analyses were conducted on 55 water samples from springs, boreholes and surface water to identify potential sources of groundwater salinity and analyse the processes that control increasing salinity. The total dissolved solid (TDS) content in the groundwater ranged from 400 to 41 000 mg/l, and high TDS values were commonly associated with shallow water tables and flow‐through and discharge zones in unconfined aquifers. Various groundwater contributions from rainwater, agricultural irrigation, river water infiltration and lateral inflows from mountains were identified by major ions and δD and δ18O. In general, HCO3? and SO42? were the dominant anions in groundwater with a salinity of <2500 mg/l, whereas Cl? and SO42? were the dominant anions in groundwater with a salinity of >2500 mg/l. The major ion concentrations indicated that mineral weathering, including carbonate and evaporite dissolution, primarily affected groundwater salinity in recharge areas. Evapotranspiration controlled the major ion concentration evolution and salinity distribution in the unconfined groundwaters in the flow‐through and discharge areas, although it had a limited effect on groundwater in the recharge areas and confined aquifers. Agricultural irrigation increased the water table and enhanced evapotranspiration in the oasis areas of the basin. TDS and Cl became more concentrated, but H and O isotopes were not enriched in the irrigation district, indicating that transpiration dominated the increasing salinity. For other places in the basin, as indicated by TDS, Cl, δD and δ18O characteristics, evaporation, transpiration and water–rock interactions dominated at different hydrogeological zones, depending on the plant coverage and hydrogeological conditions. Groundwater ages of 3H, and δD and δ18O compositions and distributions suggest that most of the groundwaters in Dunhuang Basin have a paleometeoric origin and experienced a long residence time. These results can contribute to groundwater management and future water allocation programmes in the Dunhuang Basin. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

8.
Water use efficiency (WUE) links carbon and water exchanges between farmlands and the atmosphere. Understanding the variation and attribution of WUE is essential to reveal the physiological and ecological adaptation mechanisms of crops to the changing environment, and to better allocate, regulate and conserve water resources. However, few studies on the variation and attribution of WUE have been conducted in irrigated arid or semi-arid farmlands. Therefore, in this study, water and carbon fluxes were measured using eddy covariance systems in two farmlands (one sunflower field and one maize field) in a semi-arid irrigation district in China. It was found that the average WUE of sunflower during its full growth period was 1.72 g C kg−1 H2O, much lower than that of maize (4.07 g C kg−1 H2O). At each growth stage, the WUE of both crops were negatively correlated with vapour pressure deficit (VPD), net radiation (Rn) and soil water content (SWC). The negative correlations could be attributed to the arid meteorological condition and the relatively abundant soil moisture due to irrigation and shallow groundwater levels. VPD was the main factor affecting WUE, followed by Rn and SWC. It was also found that the response of WUE to crop leaf area index (LAI) and to canopy conductance (gc) depended on the VPD ranges: when VPD increased, the response of WUE to LAI and to gc decreased. Our findings could improve the understanding of the coupling effect of water and carbon fluxes over farmland ecosystems in arid and semi-arid irrigation areas and help improve agricultural production and save water resources in such areas.  相似文献   

9.
Components of the energy budget were measured continuously above a 300‐year‐old temperate mixed forest at the Changbaishan site, northeastern China, from 1 January to 31 December 2003, as a part of the ChinaFlux programme. The albedo values above the canopy were lower than most temperate forests, and the values for snow‐covered canopy were over 50% higher than for the snow‐free canopy. In winter, net radiation Rn was generally less than 5% of the summer value due to high albedo and low incoming solar radiation. The annual mean latent heat LE was 37·5 W m?2, accounting for 52% of Rn. The maximum daily evaporation was about 4·6 mm day?1 in summer. Over the year, the accumulated precipitation was 578 mm; this compares with 493 mm of evapotranspiration, which shows that more than 85% of water was returned to the atmosphere through evapotranspiration. The LE was strongly affected by the transpiration activity and increased quickly as the broadleaved trees began to foliate. The sensible heat H dropped at that time, although Rn increased. Consequently, the seasonal variation in the Bowen ratio β was clearly U‐shaped, and the minimum value (0·1) occurred on a sunny day just after rain, when most of the available energy was used for evapotranspiration. Negative β values occurred occasionally in the non‐growing season as a result of intensive radiative cooling and the presence of water on the surface. The β was very high (up to 13·0) in snow‐covered winter, when evapotranspiration was small due to low surface temperature and available soil water. Vegetation phenology and soil moisture were the key variables controlling the available energy partitioning between H and LE. Energy budget closure averaged better than 86% on a half‐hourly basis, with slightly greater closure on a daily basis. The degree of closure showed a dependence on friction velocity u*. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

10.
Shallow groundwater plays a key role in agro‐hydrological processes of arid areas. Groundwater often supplies a necessary part of the water requirement of crops and surrounding native vegetation, such as groundwater‐dependent ecosystems. However, the impact of water‐saving irrigation on cropland water balance, such as the contribution of shallow groundwater to field evapotranspiration, requires further investigation. Increased understanding of quantitative evaluation of field‐scale water productivity under different irrigation methods aids policy and decision‐making. In this study, high‐resolution water table depth and soil water content in field maize were monitored under conditions of flood irrigation (FI) and drip irrigation (DI), respectively. Groundwater evapotranspiration (ETg) was estimated by the combination of the water table fluctuation method and an empirical groundwater–soil–atmosphere continuum model. The results indicate that daily ETg at different growth stages varies under the two irrigation methods. Between two consecutive irrigation events of the FI site, daily ETg rate increases from zero to greater than that of the DI site. Maize under DI steadily consumes more groundwater than FI, accounting for 16.4% and 14.5% of ETa, respectively. Overall, FI recharges groundwater, whereas DI extracts water from shallow groundwater. The yield under DI increases compared with that under FI, with less ETa (526 mm) compared with FI (578 mm), and irrigation water productivity improves from 3.51 kg m?3 (FI) to 4.58 kg m?3 (DI) through reducing deep drainage and soil evaporation by DI. These results highlight the critical role of irrigation method and groundwater on crop water consumption and productivity. This study provides important information to aid the development of agricultural irrigation schemes in arid areas with shallow groundwater.  相似文献   

11.
Woody plant encroachment is a global phenomenon whereby shrubs or trees replace grasses. The hydrological consequences of this ecological shift are of broad interest in ecohydrology, yet little is known of how plant and intercanopy patch dynamics, distributions, and connectivity influence catchment-scale responses. To address this gap, we established research catchments in the Sonoran and Chihuahuan Deserts (near Green Valley, Arizona and near Las Cruces, New Mexico, respectively) that represent shrub encroachment in contrasting arid climates. Our main goals in the coordinated observations were to: (a) independently measure the components of the catchment water balance, (b) deploy sensors to quantify the spatial patterns of ecohydrological processes, (c) use novel methods for characterizing catchment properties, and (d) assess shrub encroachment impacts on ecohydrological processes through modelling studies. Datasets on meteorological variables; energy, radiation, and CO2 fluxes; evapotranspiration; soil moisture and temperature; and runoff at various scales now extend to nearly 10 years of observations at each site, including both wet and dry periods. Here, we provide a brief overview of data collection efforts and offer suggestions for how the coordinated datasets can be exploited for ecohydrological inferences and modelling studies. Given the representative nature of the catchments, the available databases can be used to generalize findings to other catchments in desert landscapes.  相似文献   

12.
Water availability is one of the key environmental factors that control ecosystem functions in temperate forests. Changing climate is likely to alter the ecohydrology and other ecosystem processes, which affect forest structures and functions. We constructed a multi‐year water budget (2004–2010) and quantified environmental controls on an evapotranspiration (ET) in a 70‐year‐old mixed‐oak woodland forest in northwest Ohio, USA. ET was measured using the eddy‐covariance technique along with precipitation (P), soil volumetric water content (VWC), and shallow groundwater table fluctuation. Three biophysical models were constructed and validated to calculate potential ET (PET) for developing predictive monthly ET models. We found that the annual variability in ET was relatively stable and ranged from 578 mm in 2009 to 670 mm in 2010. In contrast, ET/P was more variable and ranged from 0.60 in 2006 to 0.96 in 2010. Mean annual ET/PET_FAO was 0.64, whereas the mean annual PET_FAO/P was 1.15. Annual ET/PET_FAO was relatively stable and ranged from 0.60 in 2005 to 0.72 in 2004. Soil water storage and shallow groundwater recharge during the non‐growing season were essential in supplying ET during the growing season when ET exceeded P. Spring leaf area index (LAI), summer photosynthetically active radiation, and autumn and winter air temperatures (Ta) were the most significant controls of monthly ET. Moreover, LAI regulated ET during the whole growing season and higher temperatures increased ET even during dry periods. Our empirical modelling showed that the interaction of LAI and PET explained >90% of the variability in measured ET. Altogether, we found that increases in Ta and shifts in P distribution are likely to impact forest hydrology by altering shallow groundwater fluctuations, soil water storage, and ET and, consequently, alter the ecosystem functions of temperate forests. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

13.
Seasonal changes in the water and energy exchanges over a pine forest in eastern Siberia were investigated and compared with published data from a nearby larch forest. Continuous observations (April to August 2000) were made of the eddy‐correlation sensible heat flux and latent heat flux above the canopy. The energy balance was almost closed, although the sum of the turbulent fluxes sometimes exceeded the available energy flux (Rn ? G) when the latent heat flux was large; this was related to the wind direction. We examined the seasonal variation in energy balance components at this site. The seasonal variation and magnitude of the sensible heat flux (H) was similar to that of the latent heat flux (λE), with maximum values occurring in mid‐June. Consequently, the Bowen ratio was around 1·0 on many days during the study period. On some clear days just after rainfall, λE was very large and the sum of H and λE exceeded Rn ? G. The evapotranspiration rate above the dry canopy from May to August was 2·2 mm day?1. The contributions of understory evapotranspiration (Eu) and overstory transpiration (Eo) to the evapotranspiration of the entire ecosystem (Et) were both from 25 to 50% throughout the period analysed. These results suggest that Eu plays a very important role in the water cycle at this site. From snowmelt through the tree growth season (23 April to 19 August 2000), the total incoming water, comprised of the sum of precipitation and the water equivalent of the snow at the beginning of the melt season, was 228 mm. Total evapotranspiration from the forest, including interception loss and evaporation from the soil when the canopy was wet, was 208–254 mm. The difference between the incoming and outgoing amounts in the water balance was from +20 to ?26 mm. The water and energy exchanges of the pine and larch forest differed in that λE and H increased slowly in the pine forest, whereas λE increased rapidly in the larch forest and H decreased sharply after the melting season. Consequently, the shape of the Bowen ratio curves at the two sites differed over the period analysed, as a result of the differences in the species in each forest and in soil thawing. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

14.
Abstract

Abstract Water balances for a re-vegetated xerophyte shrub (Caragana korshinskii) area were compared to that of a bare surface area by using auto-weighing type lysimeters during the 1990–1995 growing seasons at the southeast Tengger Desert, Shapotou, China. The six-year experiment displayed how major daily water balance components might vary for a bare and a re-vegetated sand dune area. Evapotranspiration from the C. korshinskii lysimeter represented a major part of the water balance. The average annual ET/P ratios varied between 69 and 142%. No seepage was observed for the vegetated lysimeter. For the bare lysimeter, on the other hand, 48 mm or 27% of observed rainfall per year occurred as seepage. These results suggest that re-vegetating large sandy areas with xerophytic shrubs could reduce soil water storage by transpiration. Also, the experimental results indicate that re-vegetating large sandy areas could significantly change groundwater recharge conditions. However, from a viewpoint of desert ecosystem reconstruction, it appears that natural rainfall can sustain xerophytic shrubs such as C. korshinskii which would reduce erosion loss of sand. However, re-vegetation has to be balanced with recharge/groundwater needs of local populations.  相似文献   

15.
In this study, we examined the year 2011 characteristics of energy flux partitioning and evapotranspiration of a sub‐alpine spruce forest underlain by permafrost on the Qinghai–Tibet Plateau (QPT). Energy balance closure on a half‐hourly basis was H + λE = 0.81 × (Rn ? G ? S) + 3.48 (W m?2) (r2 = 0.83, n = 14938), where H, λE, Rn, G and S are the sensible heat, latent heat, net radiation, soil heat and air‐column heat storage fluxes, respectively. Maximum H was higher than maximum λE, and H dominated the energy budget at midday during the whole year, even in summer time. However, the rainfall events significantly affected energy flux partitioning and evapotranspiration. The mean value of evaporative fraction (Λ = λE/(λE + H)) during the growth period on zero precipitation days and non‐zero precipitation days was 0.40 and 0.61, respectively. The mean daily evapotranspiration of this sub‐alpine forest during summer time was 2.56 mm day?1. The annual evapotranspiration and sublimation was 417 ± 8 mm year?1, which was very similar to the annual precipitation of 428 mm. Sublimation accounted for 7.1% (30 ± 2 mm year?1) of annual evapotranspiration and sublimation, indicating that the sublimation is not negligible in the annual water balance in sub‐alpine forests on the QPT. The low values of the Priestley–Taylor coefficient (α) and the very low value of the decoupling coefficient (Ω) during most of the growing season suggested low soil water content and conservative water loss in this sub‐alpine forest. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

16.
Irrigation of agricultural oases is the main water consumer in semi‐arid and arid regions of Northwestern China. The accurate estimation of evapotranspiration (ET) on the oases is extremely important for evaluating water use efficiency so as to reasonably allocate water resources, particularly in semi‐arid and arid areas. In this study, we integrated the soil moisture information into surface energy balance system (SEBS) for improving irrigated crop water consumption estimation. The new approach fed with the moderate resolution imaging spectro‐radiometer images mapped spatiotemporal ET on the oasis in the middle reach of the Heihe river. The daily ET outputs of the new approach were compared with those of the original SEBS using the eddy correlation observations, and the results demonstrate that the modified SEBS remedied the shortcoming of general overestimating ET without regard to soil water stress. Meanwhile, the crop planting structure and leaf area index spatiotemporal distribution in the studied region were derived from the high‐resolution Chinese satellite HJ‐1/CCD images for helping analyse the pattern of the monthly ET (ETmonthly). The results show that the spatiotemporal variation of ETmonthly is closely related to artificial irrigation and crop growth. Further evaluation of current irrigation water use efficiency was conducted on both irrigation district scale and the whole middle reach of the Heihe river. The results reveal that the average fraction of consumed water on irrigation district scale is 57% in 2012. The current irrigation water system is irrational because only 52% of the total irrigated amount was used to fulfil plant ET requirement and the rest of the irrigation water recharged into groundwater in the oasis in 2012. However, in view of the whole middle reach of the Heihe river, the irrigation water use efficiency could reach to 66% in 2012. But pumping groundwater for reused irrigation wastes mostly energy instead of water. An improved irrigation water allocation system according to actual ET requirement is needed to increase irrigation efficiency per cubic meter water resource in an effort to save both water and energy. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

17.

Six species of more than 20-year-old desert woody plants in the oasis-desert ecotone were selected for study. The results showed that: (1) in different growing seasons δ13C values of assimilating organ varied between -14%. and -16%. for Haloxylon ammodendron (HA),-14%. - -15%. for Calligonum mongolicum (CM) and -25%. - -28%. for Caragana korshinskii (CK), Nitraria sphaerocarpa (NS) and Hedysarum scoparium (HS). (2) The net photosynthetic rate (Pn) of HA and CM was significantly higher than those of the other species. With the decrease in Pn for the six species, their intercellular CO2 concentration increased, but stomatal limitation value decreased under the intensive light. At the same time, the photochemical efficiency of PS II dropped to different degrees. (3) The CO2 enrichment experiment demonstrated that, Pn of HA and CM increased to different extent under 450 μmol/mol, but their Pn reduced or approximated to the current condition under 650 μmol/mol. Under 450 μmol/mol the efficiency for solar energy utilization of CK and HS significantly reduced and under 650 μmol/mol their respiration rate exceeded photosynthesis rate. It can be concluded that HA and CM have some function of pathway for C4 but the other three species have the function for C3. The decline in their Pn is mainly caused by non-stomatal factors. HA, CM, CK and HS exhibited photoinhibition, which disappeared in a short time. This is a kind of positive readjustment to adapting to the desert environment. HA and CM can adapt to the high CO2 environment, but CK and HS cannot. With the rise in atmospheric CO2 concentration and climate warming, the latter two species in the oasis-desert ecotone may be gradually degraded or even disappear.

  相似文献   

18.
Haloxylon ammodendron is a desert shrub used extensively in China for restoring degraded dry lands. An understanding of the water source used by H. ammodendron plantations is critical achieving sustainable vegetation restoration. We measured mortality, shoot size, and rooting depth in 5‐, 10‐, 20‐, and 40‐year‐old H. ammodendron plantations. We examined stable isotopic ratios of oxygen (δ18O) in precipitation, groundwater, and soil water in different soil layers and seasons, and in plant stem water to determine water sources at different shrub ages. We found that water acquisition patterns in H. ammodendron plantations differed with plantation age and season. Thus, the main water source for 5‐year‐old shrubs was shallow soil water. Water sources of 10‐year‐old shrubs shifted depending on the soil water conditions during the season. Although their tap roots could absorb deep soil water, the plantation main water sources were from soil water, and about 50% of water originated from shallow and mid soil. This pattern might occur because main water sources in these plantations were changeable over time. The 20‐ and 40‐year‐old shrubs acquired water mainly from permanent groundwater. We conclude that the main water source of a young H. ammodendron plantation was soil water recharged by precipitation. However, when roots reached sufficient depth, water originated mainly from the deep soil water, especially in the dry season. The deeply rooted 20‐ and 40‐year‐old shrubs have the ability to exploit a deep and reliable water source. To achieve sustainability in these plantations, we recommend a reduction in the initial density of H. ammodendron in the desert‐oasis ecotone to decelerate the consumption of shallow soil water during plantation establishment.  相似文献   

19.
Pan evaporation (Ep) is an important indicator of water and energy and the decline of Ep has been reported in many regions over the last decades. The climate and Ep are dependent on each other. In this study, the temporal trends of Ep and main Ep drivers, namely mean air temperature (Ta), wind speed (u), global solar radiation (Rs), net long‐wave radiation(Rnl) and vapour pressure deficit (D) from 1970 to 2012, were calculated on the basis of 26 meteorological stations on the Tibetan Plateau. The arithmetic average of Ep from 26 stations decreased with the rate of ?11.91 mm a?2; the trends of Rs, Rnl, Ta, u and D were ?1.434 w m?2 decade?1, 0.2511 w m?2 decade?1, 0.3590°C decade?1, ?0.2376 m s?1 decade?1 and 9.523 Pa decade?1, respectively. The diffuse irradiance is an essential parameter to model Ep and quantify the contribution of climatic factors to changing Ep. 60 724 observations of Rs and diffuse solar irradiance (Rd) from seven of the 26 stations were used to develop the correlation between the diffuse fraction (Rd/Rs), and the clearness index (Rs/Ro). On the basis of the estimation of the diffuse component of Rs and climatic data, we modified the PenPan model to estimate Chinese micro‐pan evaporation (Ep) and assess the attribution of Ep dynamics using partial derivatives. The results showed that there was a good agreement between the observed and calculated daily Ep values. The observed decrease in Ep was mostly due to declining wind speed (?13.7 mm a?2) with some contributions from decreasing solar irradiance (?3.1 mm a?2); and the increase of temperature had a large positive effect (4.55 mm a?2) in total whilst the increase of Rnl had insignificant effect (0.35 mm a?2) on Ep rates. The change of Ep is the net result of all the climatic variables. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

20.
Hong Xie  Xuan Zhu 《水文研究》2013,27(25):3685-3693
Evapotranspiration is an important component of the water and energy balance. It is dependent on climate. Precipitation, solar radiation, temperature, humidity, and wind all contribute to the rate of evapotranspiration. In this study, the temporal trends of reference evapotranspiration (ETref) and four main ETref drivers, namely, mean air temperature (Ta), wind speed (u2), net radiation (Rn) and actual vapour pressure (ea) from 1970 to 2009, were calculated based on 75 meteorological stations on the Tibetan Plateau. The results showed that the ETref on the Tibetan Plateau decreased on average by 0.6909 mm a‐1a‐1 from 1970 to 2009. Ta and ea showed an increasing trend, whereas u2 and Rn exhibited a decreasing trend. To explore the underlying causes of the ETref variation, an attribution analysis was performed to quantify the contribution of Ta, u2, Rn and ea, which showed that the changes in u2, Rn and ea produced the negative effect, whereas Ta produced the positive effect on ETref rates. The changes in u2 were found to produce the largest decrease (?0.7 mm) in ETref, followed by ea (?0.4 mm) and Rn (?0.1 mm). Although the significant increase in Ta had a large positive effect (0.51 mm) on ETref rates, changes in the other three variables each reduced ETref rates, resulting in an overall negative trend in ETref. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

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