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1.
Estimation of reference evapotranspiration (ET0) in urban areas is challenging but essential in arid urban climates. To evaluate ET0 in an urban environment and non-urban areas, air temperature and relative humidity were measured at five different sites across the arid city of Isfahan, Iran, over 4 years. Wind speed and sunshine hours were obtained from an urban surrounding weather station over the same period and used to estimate ET0. Calculated ET0 was compared with satellite-based ET0 retrieved from the MOD16A2 PET product. Although MODIS PET was strongly correlated with the Valiantzas equation, it overestimated ET0 and showed average accuracy (r = 0.93–0.94, RMSE = 1.18–1.28 mm/day, MBE = 0.73–0.84 mm/day). The highest ET0 differences between an urban green space and a non-urban area were 1.1 and 0.87 mm/day, which were estimated by ground measurements and MODIS PET, respectively. The sensitivity of ET0 to wind speed and sunshine hours indicated a significant effect on cumulative ET0 at urban sites compared to the non-urban site, which has a considerable impact on the amount of irrigation required in those areas. Although MODIS PET requires improvement to accurately reflect field level microclimate conditions affecting ET0, it is beneficial to hydrological applications and water resource managers especially in areas where data is limited. In addition, our results indicated that using limited data methods or meteorological data from regional weather stations, leads to incorrect estimation of ET0 in urban areas. Therefore, decision-makers and urban planners should consider the importance of precisely estimating ET0 to optimize management of urban green space irrigation, especially in arid and semi-arid climates such as the city of Isfahan.  相似文献   

2.
Abstract

The Blaney-Criddle (BC) temperature-based equation is used in areas where the complete weather data to estimate reference evapotranspiration (ET0) by the Penman-Monteith FAO-56 (PMF-56) standard model is complex. In this study, the BC equation was first tested and calibrated against the ET0 values computed by the PMF-56 method using data from 17 weather stations in arid regions of Iran. Then, geographical information systems (GIS)-based spatially-distributed maps of ET0 were prepared by means of geographic/topographic factors derived from a digital elevation model (DEM) for all months, separately. The results indicate that the original BC equation overestimated PMF-56 ET0 by 4% at the study sites. The BC equation produced closer ET0 estimates to the PMF-56 method after it was calibrated. The error rate of <3% for the spatial modelling approach suggests that the developed ET0 maps are reliable.

Editor D. Koutsoyiannis; Associate editor D. Yang

Citation Tabari, H., Hosseinzadeh Talaee, P., and Shifteh Some'e, B., 2013. Spatial modelling of reference evapotranspiration using adjusted Blaney-Criddle equation in an arid environment. Hydrological Sciences Journal, 58 (2), 408–420.  相似文献   

3.
Sustainable water management in semi-arid agriculture practices requires quantitative knowledge of water fluxes within the soil-vegetation-atmosphere system. Therefore, we used stable-isotope approaches to evaluate evaporation (Ea), transpiration (Ta), and groundwater recharge (R) at sites in Senegal's Groundnut basin and Ferlo Valley pasture region during the pre-monsoon, monsoon, and post-monsoon seasons of 2021. The approaches were based upon (i) the isothermal evaporation model (for quantifying Ea); (ii) water and isotope mass balances (to partition Ea and Ta for groundnut and pasture); and (iii) the piston displacement method (for estimating R). Ea losses derived from the isothermal evaporation model corresponded primarily to Stage II evaporation, and ranged from 0.02 to 0.09 mm d−1 in the Groundnut basin, versus 0.02–0.11 mm d−1 in Ferlo. At the groundnut site, Ea rates ranged from 0.01 to 0.69 mm d−1; Ta was in the range 0.55–2.29 mm d−1; and the Ta/ETa ratio was 74%–90%. At the pasture site, the ranges were 0.02–0.39 mm d−1 for Ea; 0.9–1.69 mm d−1 for Ta; and 62–90% for Ta/ETa. The ETa value derived for the groundnut site via the isotope approach was similar to those from eddy covariance measurements, and also to the results from the previous validated HYDRUS-1D model. However, the HYDRUS-1D model gave a lower Ta/ETa ratio (23.2%). The computed groundwater recharge for the groundnut site amounted to less than 2% of the local annual precipitation. Recommendations are made regarding protocols for preventing changes to isotopic compositions of water in samples that are collected in remote arid regions, but must be analysed days later. The article ends with suggestions for studies to follow up on evidence that local aquifers are being recharged via preferential pathways.  相似文献   

4.
Evapotranspiration (ET) is one of the basic components of the hydrologic cycle and is essential for estimating irrigation water requirements. In this study, an artificial neural network (ANN) model for reference evapotranspiration (ET0) calculation was investigated. ANNs were trained and tested for arid (west), semi‐arid (middle) and sub‐humid (east) areas of the Inner Mongolia district of China. Three or four climate factors, i.e. air temperature (T), relative humidity (RH), wind speed (U) and duration of sunshine (N) from 135 meteorological stations distributed throughout the study area, were used as the inputs of the ANNs. A comparison was conducted between the estimates provided by the ANNs and by multilinear regression (MLR). The results showed that ANNs using the climatic data successfully estimated ET0 and the ANNs simulated ET0 better than the MLRs. The ANNs with four inputs were more accurate than those with three inputs. The errors of the ANNs with four inputs were lower (with RMSE of 0·130 mm d?1, RE of 2·7% and R2 of 0·986) in the semi‐arid area than in the other two areas, but the errors of the ANNs with three inputs were lower in the sub‐humid area (with RMSE of 0·21 mm d?1, RE of 5·2% and R2 of 0·961. For the different seasons, the results indicated that the highest errors occurred in September and the lowest in April for the ANNs with four inputs. Similarly, the errors were higher in September for the ANNs with three inputs. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

5.
Analysis of spatial and temporal variations of reference evapotranspiration (ETo) is important in arid and semi‐arid regions where water resources are limited. The main aim of this study was to analyse the spatial distribution and the annual, seasonal and monthly trends of the Penman–Monteith ETo for 21 stations in the arid and semi‐arid regions of Iran. Three statistical tests the Mann‐Kendall, Sen's slope estimator and linear regression were used for the analysis. The analysis revealed that ETo increased from January to July and deceased from July to December at almost all stations. Additionally, higher annual ETo values were found in the southeast of the study region and lower values in the northwest of the region. Although the results showed both positive and negative trends in annual ETo series, ETo generally increased, significantly so in six (～30%) of the stations. Analysis of the impacts of meteorological variables on the temporal trends of ETo indicated that the increasing trend of ETo was most likely due to a significant increase in minimum air temperature, while decreasing trend of ETo was mainly caused by a significant decrease in wind speed. At the sites where increasing ETo trends were statistically significant, the rate of increase varied from (+)8·36 mm/year at Mashhad station to (+)31·68 mm/year at Iranshahr station. On average, an increasing trend of (+)4·42 mm/year was obtained for the whole study area during the last four decades. Seasonal and monthly ETo have also tended to increase at the majority of the stations. The greatest numbers of significant trends were observed in winter on the seasonal time‐scale and in September on the monthly time‐scale. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

6.
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   

7.
Accurate estimation of evapotranspiration (ET) is essential in water resources management and hydrological practices. Estimation of ET in areas, where adequate meteorological data are not available, is one of the challenges faced by water resource managers. Hence, a simplified approach, which is less data intensive, is crucial. The FAO‐56 Penman–Monteith (FAO‐56 PM) is a sole global standard method, but it requires numerous weather data for the estimation of reference ET. A new simple temperature method is developed, which uses only maximum temperature data to estimate ET. Ten class I weather stations data were collected from the National Meteorological Agency of Ethiopia. This method was compared with the global standard PM method, the observed Piche evaporimeter data, and the well‐known Hargreaves (HAR) temperature method. The coefficient of determination (R2) of the new method was as high as 0.74, 0.75, and 0.91, when compared with that of PM reference evapotranspiration (ETo), Piche evaporimeter data, and HAR methods, respectively. The annual average R2 over the ten stations when compared with PM, Piche, and HAR methods were 0.65, 0.67, and 0.84, respectively. The Nash–Sutcliff efficiency of the new method compared with that of PM was as high as 0.67. The method was able to estimate daily ET with an average root mean square error and an average absolute mean error of 0.59 and 0.47 mm, respectively, from the PM ETo method. The method was also tested in dry and wet seasons and found to perform well in both seasons. The average R2 of the new method with the HAR method was 0.82 and 0.84 in dry and wet seasons, respectively. During validation, the average R2 and Nash–Sutcliff values when compared with Piche evaporation were 0.67 and 0.51, respectively. The method could be used for the estimation of daily ETo where there are insufficient data. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

8.
Groundwater provides an important source of water for maize cultivation where the water table is shallow in the semi-arid Hailiutu River catchment of the Maowusu Desert on the Erdos Plateau in Northwest China. A HYDRUS-1D model of the unsaturated flow beneath a maize (Zea mays L.) field was calibrated and validated with measured soil water contents at various depths during the maize growing period from 30 April to 1 October 2011, and from 23 May to 27 September 2012, respectively. The model computed the actual maize evapotranspiration (ETa) as 580 mm during the whole growing period from 30 April to 1 October 2011. The groundwater contribution to ETa was calculated to be 220 mm, accounting for 38% of maize water use during the growing season in 2011. When the groundwater level drops below a depth of 157 cm, maize can no longer use groundwater for transpiration. The irrigation water requirement increases with the increase of groundwater table depth. These results are very important for managing crop irrigation in the area.
EDITOR D. Koutsoyiannis

ASSOCIATE EDITOR L. Ruiz  相似文献   

9.
Different satellite-based radiation (Makkink) and temperature (Hargreaves-Samani, Penman-Monteith temperature, PMT) reference evapotranspiration (ETo) models were compared with the FAO56-PM method over the Cauvery basin, India. Maximum air temperature (Tmax) required in the ETo models was estimated using the temperature–vegetation index (TVX) and an advanced statistical approach (ASA), and evaluated with observed Tmax obtained from automatic weather stations. Minimum air temperature (Tmin) was estimated using ASA. Land surface temperature was employed in the ETo models in place of air temperature (Ta) to check the potency of its applicability. The results suggest that the PMT model with Ta as input performed better than the other ETo models, with correlation coefficient (r), averaged root mean square error (RMSE) and mean bias error (MBE) of 0.77, 0.80 mm d?1 and ?0.69 for all land cover classes. The ASA yielded better Tmax and Tmin values (r and RMSE of 0.87 and 2.17°C, and 0.87 and 2.27°C, respectively).  相似文献   

10.
F. Ashkar 《水文科学杂志》2013,58(6):1092-1106
Abstract

The potential is investigated of the generalized regression neural networks (GRNN) technique in modelling of reference evapotranspiration (ET0) obtained using the FAO Penman-Monteith (PM) equation. Various combinations of daily climatic data, namely solar radiation, air temperature, relative humidity and wind speed, are used as inputs to the ANN so as to evaluate the degree of effect of each of these variables on ET0. In the first part of the study, a comparison is made between the estimates provided by the GRNN and those obtained by the Penman, Hargreaves and Ritchie methods as implemented by the California Irrigation Management System (CIMIS). The empirical models were calibrated using the standard FAO PM ET0 values. The GRNN estimates are also compared with those of the calibrated models. Mean square error, mean absolute error and determination coefficient statistics are used as comparison criteria for the evaluation of the model performances. The GRNN technique (GRNN 1) whose inputs are solar radiation, air temperature, relative humidity and wind speed, gave mean square errors of 0.058 and 0.032 mm2 day?2, mean absolute errors of 0.184 and 0.127 mm day?1, and determination coefficients of 0.985 and 0.986 for the Pomona and Santa Monica stations (Los Angeles, USA), respectively. Based on the comparisons, it was found that the GRNN 1 model could be employed successfully in modelling the ET0 process. The second part of the study investigates the potential of the GRNN and the empirical methods in ET0 estimation using the nearby station data. Among the models, the calibrated Hargreaves was found to perform better than the others.  相似文献   

11.
Abstract

The spatio-temporal variations of reference crop evapotranspiration (ETref) reflect the combined effects of meteorological variables, primarily wind speed, relative humidity, net radiation and air temperature. This study investigated the spatial distribution and temporal trends of ETref (calculated by the FAO-56 Penman-Monteith equation), pan evaporation (Epan) and pan coefficient (Kp) in a 140?×?103 km2 semi-humid to semi-arid area in China. The results show that: (i) although the spatial distributions of ETref and Epan are roughly similar and their spatial correlation is high over the growing season, Kp varied considerably in space due to high humidity in the east of the region and low humidity in the southwest; (ii) the monthly variations of ETref and Epan are similar to that of net radiation and opposite to that of relative humidity, while the monthly variation of Kp is similar to that of relative humidity and opposite to that of wind speed, and the long-term trend is slightly increasing for ETref and Epan, while significantly (10% significance level) increasing for Kp; and (iii) generally, the time series of ETref and Epan from 1951 to 2001 could be divided into three phases due to variations of meteorological variables.

Citation Liang, L.-Q., Li, L.-J. & Liu, Q. (2011) Spatio-temporal variations of reference crop evapotranspiration and pan evaporation in the West Songnen Plain of China. Hydrol. Sci. J. 56(7), 1300–1313.  相似文献   

12.
The Western Boreal Plain of North Central Alberta comprises a mosaic of wetlands and aspen (Populus tremuloides) dominated uplands where precipitation (P) is normally exceeded by evapotranspiration (ET). As such these systems are highly susceptible to the climatic variability that may upset the balance between P and ET. Above canopy evapotranspiration (ETC) and understory evapotranspiration (ETB) were examined using the eddy covariance technique situated at 25.5 m (7.5 m above tree crown) and 4.0 m above the ground surface, respectively. During the peak period of the growing seasons (green periods), ETC averaged 3.08 mm d?1 and 3.45 mm d?1 in 2005 and 2006, respectively, while ETB averaged 1.56 mm d?1 and 1.95 mm d?1. Early in the growing season, ETB was equal to or greater than ETC once understory development had occurred. However, upon tree crown growth, ETB was lessened due to a reduction in available energy. ETB ranged from 42 to 56% of ETC over the remainder of the snow‐free seasons. Vapour pressure deficit (VPD) and soil moisture (θ) displayed strong controls on both ETC and ETB. ETC responded to precipitation events as the developed tree crown intercepted and held available water which contributed to peak ETC following precipitation events >10 mm. While both ETC and ETB were shown to respond to VPD, soil moisture in the rooting zone is shown to be the strongest control regardless of atmospheric demand. Further, soil moisture and tension data suggest that rooting zone soil moisture is controlled by the redistribution of soil water by the aspen root system. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

13.
ABSTRACT

The Hargreaves method provides reference evapotranspiration (ETo) estimates when only air temperature data are available, although it requires previous local calibration for an acceptable performance. This method was evaluated using the data from 71 meteorological stations in the Seolma-cheon basin (8.48 km2), South Korea, comparing daily estimates against those from the Penman‐Monteith (PM) method, which was used as the standard. To estimate reference ETo more exactly, considering the climatological characteristics in South Korea, parameter regionalization of the Hargreaves equation is carried out. First, the modified Hargreaves equation is presented after an analysis of the relationship between solar radiation and temperature. Second, parameter (KET) optimization of the regional calibration of the Hargreaves equation (RCH) is performed using the PM method and the modified equation at 71 meteorological stations. Next, an application was carried out to evaluate the evapotranspiration methods (PM, original Hargreaves and RCH) in the SWAT (Soil and Water Assessment Tool) model by comparing these with the measured actual evapotranspiration (AET) in the basin. The SWAT model was calibrated using 3 years (2007–2009) of daily streamflow at the watershed outlet and 3 years (2007–2009) of daily AET measured at a mixed forest. The model was validated with 3 years (2010‐2012) of streamflow and AET. RCH will contribute to a better understanding of evapotranspiration of an ungauged watershed in areas where meteorological information is scarce.
EDITOR D. Koutsoyiannis ASSOCIATE EDITOR Not assigned  相似文献   

14.
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   

15.
This study explores the feasibility of an entirely satellite remote sensing (RS)‐based hydrologic budget model for a ground data‐constrained basin, the Rufiji basin in Tanzania, from the balance of runoff (Q), precipitation (P), storage change (ΔS), and evapotranspiration (ET). P was determined from the Tropical Rainfall Measuring Mission, ΔS from the Gravity Recovery and Climate Experiment, and ET from the Moderate Resolution Imaging Spectroradiometer, the surface radiation budget, and the Atmosphere Infrared Radiation Sounder. Q was estimated as a residual of the water balance and tested against measured Q for a sub‐basin of the Rufiji (the Usangu basin) where ground measurements were available (R2 = 0.58, slope = 1.9, root mean square error = 29 mm/month, bias = 14%). We also tested a geographical information system (GIS)‐driven (ArcCN‐runoff) runoff model (R2 = 0.64, slope = 0.43, root mean square error = 39 mm/month). We conducted an error propagation analysis from each of the model's hydrologic components (P, ET, and ΔS). We find that the RS‐based model amplitude is most sensitive to ET and slightly less so to P, whereas the model's seasonal trends are most sensitive to ?S. Although RS–GIS‐driven models are becoming increasingly used, our results indicate that long‐term water resource assessment policy and management may be more appropriate than ‘instantaneous’ or short‐term water resource assessment. However, our analyses help develop a series of tools and techniques to progress our understanding of RS–GIS in water resource management of data‐constrained basins at the level of a water resource manager. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

16.
ABSTRACT

The impact of climate variables on monthly reference evapotranspiration (ETo) is a critical issue in water resources management and irrigation planning. The spatio-temporal contribution of climate variables to ETo in the Pearl River Basin (PRB), China, from 1960 to 2016 were calculated based on sensitivity and relative change of each climatic variable. The results show that annual ETo total decreased by 1.64% and diminished in magnitude from the southeast to the northwest. Sunshine duration, wind speed and relative humidity decreased by 15.5%, 7.4%, and 4.0%, respectively, while average temperature increased by 4.25%. The ETo showed a positive sensitivity to all variables except relative humidity, which showed a negative sensitivity. Sunshine duration had the highest contribution of ?4.26%, and the overall decrease in ETo was mainly caused by the declines in sunshine duration and wind speed, which offset the positive impact of rises in average temperature and reduction in relative humidity.  相似文献   

17.
The backward‐averaged iterative two‐source surface temperature and energy balance solution (BAITSSS) model was developed to calculate evapotranspiration (ET) at point to regional scales. The BAITSSS model is driven by micrometeorological data and vegetation indices and simulates the water and energy balance of the soil and canopy sources separately, using the Jarvis model to calculate canopy resistance. The BAITSSS model has undergone limited testing in Idaho, United States. We conducted a blind test of the BAITSSS model without prior calibration for ET against weighing lysimeter measurements, net radiation, and surface temperature of drought‐tolerant corn (Zea mays L. cv. PIO 1151) in a semiarid, advective climate (Bushland, Texas, United States) in 2016. Later in the season (20 days), BAITSSS consistently overestimated ET by up to 3 mm d?1. For the entire growing season (127 days), simulated versus measured ET resulted in a 7% error in cumulative ET, RMSE = 0.13 mm h?1, and 1.70 mm d?1; r2 = 0.66 (daily) and r2 = 0.84 (hourly); MAE = 0.08 mm h?1 and 1.24 mm d?1; and MBE = 0.02 mm h?1 and 0.58 mm d?1. The results were comparable with thermally driven instantaneous ET models that required some calibration. Next, the initial soil water boundary condition was reduced, and model revisions were made to resistance terms related to incomplete cover and assumption of canopy senescence. The revisions reduced discrepancies between measured and modelled ET resulting in <1% error in cumulative ET, RMSE = 0.1 mm h?1, and 1.09 mm d?1; r2 = 0.86 (daily) and r2 = 0.90 (hourly); MAE = 0.06 mm h?1 and 0.79 mm d?1; and MBE = 0.0 mm h?1 and 0.17 mm d?1 and generally mitigated the previous overestimation. The advancement in ET modelling with BAITSSS assists to minimize uncertainties in crop ET modelling in a time series.  相似文献   

18.
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   

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.
The accurate estimation of evapotranspiration (ET) is essential for assessing water availability and requirements of regional-scale terrestrial ecosystems, and for understanding the hydrological cycle in alpine ecosystems. In this study, two large-scale weighing lysimeters were employed to estimate the magnitude and dynamics of actual evapotranspiration in a humid alpine Kobresia meadow from January 2018 to December 2019 on the northeastern Qinghai-Tibetan Plateau (QTP). The results showed that daily ETa averaged 2.24 ± 0.10 mm day −1 throughout the study period, with values of 3.89 ± 0.14 and 0.81 ± 0.06 mm day−1 during the growing season and non-growing season, respectively. The cumulative ETa during the study period was 937.39 mm, exceeding precipitation (684.20 mm) received at the site during the same period by 37%, suggesting that almost all precipitation in the lysimeters was returned to the atmosphere by evapotranspiration. Furthermore, the cumulative ETa (805.04 mm) was almost equal to the maximum potential evapotranspiration estimated by the FAO-56 reference evapotranspiration (ET0) (801.94 mm) during the growing season, but the cumulative ETa (132.25 mm) was 113.72% less than the minimum equilibrium ETeq) (282.86 mm) during the non-growing season due to the limited surface moisture in frozen soil. The crop coefficient (Kc) also showed a distinct seasonal pattern, with a monthly average of 1.01 during the growing season. Structural equation model (SEM) and boosted regression tree (BRT) show that net radiation and air temperature were the most important factors affecting daily ETa during the whole study period and growing season, but that non-growing season ETa was dominated by soil water content and net radiation. The daily Kc was dominated by net radiation. Furthermore, both ETa and Kc were also affected by aboveground biomass.  相似文献   

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