Instantaneous Evapotranspiration Research Articles

Estimating high spatiotemporal resolution evapotranspiration over a winter wheat field using an IKONOS image based complementary relationship and Lysimeter observations

Mapping high spatiotemporal resolution evapotranspiration (ET) over large areas is important for water resources planning, precision irrigation and monitoring water use efficiency. However, both traditional field measurement and aerodynamic estimation mainly focus on obtaining local ET. Remote sensing observations usually can be used to retrieve instantaneous ET at a low spatial resolution over region or global scale. Therefore, using field measurements and high resolution image data to generate high spatiotemporal resolution ET is becoming an important research direction. In this study, the complementary relationship model (CR) was tested together with meteorological data to estimate actual ET, and the results were validated by the Lysimeter observation. Furthermore, CR model combined with high resolution IKONOS data was used to estimate instantaneous field scale ET that was then transferred to daily ET. The cumulative evapotranspiration (ET) of winter wheat during the reproductive period from March through June of 2011 was 469.12mm, essentially corresponding to the annual precipitation in the study area. The highest accuracy of estimating ET by CR model with remote sensing data was in May (R2=0.863, RMSE=0.103mm). The transferred daily ET by a self-preservation of evaporative fraction (EF) approach from the CR modeling instantaneous ET was consistent with The Lysimeter measurements for all four months, March through June, 2011 (R2=0.937, RMSE=0.668mm). The experimental results demonstrate that CR model can be used to accurately estimate actual ET with both meteorological data and high resolution remote sensing data at a regional scale.

Retrieval of the surface evapotranspiration patterns in the alpine grassland–wetland ecosystem applying SEBAL model in the source region of the Yellow River, China

The source region of the Yellow River is located in the eastern of Tibetan Plateau at an altitude of 3900–5300m. Applying the method of unsupervised classification, ISODATA, we defined the land covers of the region into five types, snow cover, water body, bare soil, low coverage grassland, and middle-and-high coverage grassland. To learn the pattern of evapotranspiration (ET) at large scale is essential for the local water use and management. With surface energy balance algorithm for land (SEBAL), we retrieved ET and the energy fluxes with Landsat 5 remote sensing images of the area in the summer of 2006. It was estimated that the instantaneous evapotranspiration was highest in middle-and-high coverage grassland of the southern region at 0.31mm/h on average level, and lowest in bare soil of the northern region at 0.13mm/h on average level. The key variables in SEBAL model, including the normalized difference vegetation index, surface albedo, the surface radiation temperature, net radiation, and sensible heat fluxes were calculated and illustrated to see their patterns in the region. At final, based on the correlation analysis among the key variables, we conducted a conceptual framework of cause–effect loops involving the main factors in the process of ET. Under the framework, we described the typical situations of hydro-heat in the source region.

How representative are instantaneous evaporative fraction measurements of daytime fluxes?

Abstract. Sun-synchronous optical and thermal remote sensing is a promising technique to provide instantaneous ET (evapotranspiration) estimates during satellite overpass. The common approach to extrapolate the instantaneous estimates to values for daily or longer periods relies on the assumption that the EF (evaporative fraction, defined as the ratio of latent heat flux to surface available energy) remains nearly constant during daytime. However, there is still no consensus on the validity of the self-preservation of the EF. We use FLUXNET (a global network of eddy covariance stations) measurements to examine this self-preservation, and the conditions under which it can hold. It is found that the instantaneous EF could represent daytime EF under clear sky conditions, especially between 11:00 and 14:00 LT (local time) for all stations. However, the results show that the EF is more variable during cloudy sky conditions, so that an increase in cloud cover results in an increase in the variability of the EF during daytime.

Temporal upscaling of instantaneous evapotranspiration: An intercomparison of four methods using eddy covariance measurements and MODIS data

The quantification of land surface evapotranspiration (ET) at daily or longer time scales is of great significance in modeling the global hydrological cycle, studying climate change, and managing water resources. However, current remote sensing-based ET models can generally only provide snapshots of ET at the time of a satellite overpass and do not satisfy the expectations of hydrologists, irrigation engineers, or water resource managers concerned with practical applications. Four commonly used ET upscaling schemes, namely, the constant reference evaporative fraction (EFr) method, the constant evaporative fraction (EF) method, the constant extraterrestrial solar radiation ratio (R-p) method, and the constant observed global solar radiation ratio (R-g) method, were evaluated in this study using ground-based eddy covariance (EC) measurements and the Moderate Resolution Imaging Spectroradiometer (MODIS) derived estimates from a two-source energy balance model. Analysis was made of both closed and un-closed surface fluxes and of different assumed satellite overpass times from mid-morning to mid-afternoon. Data for the analysis were collected at the Yucheng comprehensive experimental station in Northern China, spanning the period from late April 2009 to late October 2011. The results show that all four upscaling factors from noon to mid-afternoon had a better agreement with their corresponding daily averages. Overall, the EFr (R-g) method had the best (second best) performance of the four upscaling methods. The EF method was found to significantly underestimate the daily latent heat flux (LE) and performed the worst of the four upscaling methods. With the correction of the energy imbalance of EC measurements, the R-g method's performance was improved and outperformed the EF, method in the morning. The presence of clouds either increased or decreased the BIAS but generally increased the root mean square error (RMSE) for all four upscaling methods. When the upscaling methods were applied to convert the instantaneous MODIS remote sensing estimates of ET to daily values, the accuracy of the extrapolated daily ET was controlled by the accuracy of both the remote sensing ET estimates and the upscaling methods themselves. With the insignificantly biased estimates of the instantaneous LE from the N95 model at 101 MODIS overpass times in this study, using the EF method underestimated the daily ET by 11%, while all the remaining three factors overestimated the daily ET by a range of 5%-18%. This study was conducted to provide a scientific basis for developing an operational and more accurate ET-upscaling method with easy access to data in future studies. (C) 2013 Elsevier Inc. All rights reserved.

Lysimetric evaluation of SEBAL using high resolution airborne imagery from BEAREX08

In this study, Surface Energy Balance Algorithm for Land (SEBAL) was evaluated for its ability to derive aerodynamic components and surface energy fluxes from very high resolution airborne remote sensing data acquired during the Bushland Evapotranspiration and Agricultural Remote Sensing Experiment 2008 (BEAREX08) in Texas, USA. Issues related to hot and cold pixel selection and the underlying assumptions of difference between air and surface temperature (dT) being linearly related to the surface temperature were also addressed. Estimated instantaneous evapotranspiration (ET) and other components of the surface energy balance were compared with measured data from four large precision weighing lysimeter fields, two each managed under irrigation and dryland conditions. Instantaneous ET was estimated with overall mean bias error and root mean square error (RMSE) of 0.13 and 0.15mmh−1 (23.8 and 28.2%) respectively, where relatively large RMSE was contributed by dryland field. Sensitivity analysis of the hot and cold pixel selection indicated that up to 20% of the variability in ET estimates could be attributed to differences in the surface energy balance and roughness properties of the anchor pixels. Adoption of an excess resistance to heat transfer parameter model into SEBAL significantly improved the instantaneous ET estimates.

Satellite‐based ET estimation in agriculture using SEBAL and METRIC

AbstractSurface Energy Balance Algorithms for Land (SEBAL) and Mapping EvapoTranspiration at high Resolution with Internalized Calibration (METRIC) are satellite‐based image‐processing models that calculate evapotranspiration (ET) as a residual of a surface energy balance. Both models are calibrated using inverse modelling at extreme conditions approach to develop image‐specific estimations of the sensible heat flux (H) component of the surface energy balance and to effectively remove systematic biases in net radiation, soil heat flux, radiometric temperature and aerodynamic estimates. SEBAL and METRIC express the near‐surface temperature gradient as an indexed function of radiometric surface temperature, eliminating the need for absolutely accurate surface temperature and the need for air temperature measurements. Slope and aspect functions and temperature lapsing are used in METRIC applications in mountainous terrains. SEBAL and METRIC algorithms are designed for relatively routine application by trained professionals familiar with energy balance, aerodynamics and basic radiation physics. The primary inputs for the models are short‐wave and long‐wave (thermal) images from satellite (e.g. Landsat and MODIS), a digital elevation model and ground‐based weather data measured within or near the area of interest. ET ‘maps’ (i.e. images) developed using Landsat images provide means to quantify ET on a field basis in terms of both rate and spatial distribution. METRIC takes advantage of calibration using weather‐based reference ET so that both calibration and extrapolation of instantaneous ET to 24‐h and longer periods compensate for regional advection effects where ET can exceed daily net radiation. SEBAL and METRIC have advantages over conventional methods of estimating ET using crop coefficient curves or vegetation indices in that specific crop or vegetation type does not need to be known and the energy balance can detect reduced ET caused by water shortage, salinity or frost as well as evaporation from bare soil. Copyright © 2011 John Wiley & Sons, Ltd.

A Neurofuzzy Structure Modelling Evapotranspiration in a Greenhouse Crop

This paper shows the applicability of a neurofuzzy method applied to a tomato plant (Lycopersicon esculentum, Mill), in order to obtain a model of the instantaneous evapotranspiration. Two operational dynamics (diurnal and nocturnal) are defined in a hierarchical fuzzy model by a Takagi-Sugeno (T-S) type with linear consequents. The fuzzy selector of the two dynamics is the solar radiation measure. The fuzzy C-means algorithm is used to identify the fuzzy rules premises. The hierarchical fuzzy modelling is used to obtain a multi-model of the evapotranspiration cycles. In order to simplify the model structure, the variable of vapour pressure deficit (VPD) is introduced, and thus, a physical interpretation of the interior climate is obtained. VPD helps to preserve the quality and production level in the greenhouse crop.

동아시아 지역의 플럭스 타워 관측지에 대한 MODIS 위성영상 기반의 증발산 평가

지표 증발산은 육상 생태계의 수문순환의 주요 성분으로서, 지표-대기간의 에너지 교환, 미기후, 지역의 수자원 함량, 식생의 일차생산성 등에 중요한 영향을 미친다. 증발산을 추정하기 위한 방법들 중에서 MODIS를 이용한 방법은 위성 자료만을 사용하여 넓은 지역에 대한 지속적인 증발산 모니터링이 가능하다는 장점을 갖고 있다. 본 연구에서는 MODIS 기반의 증발산 추정 알고리즘을 동아시아 지역에 적용하고, 그 신뢰도를 평가하였으며, 주요 오차요인을 분석하였다. 증발산 평가 결과 여섯 연구지역(GDK, HFK, TKY, TMK, CBS, SKT)에서는 <TEX>$r^2$</TEX>가 0.38~0.73, ME 와 RMSE가 각각 <TEX>$-44{\sim}+31W\;m^{-2}$</TEX>, <TEX>$48{\sim}111W\;m^{-2}$</TEX>로 신뢰할 만한 결과를 나타냈다. 하지만 다른 세 연구지역(HBG, QYZ, MKL)에서는 관측 값과 비교해서 차이를 나타내었고, 과소평가하는 경향을 보였다. HBG, MKL 지역은 MODIS 기상 자료 및 복사요소의 오차가 주요 원인으로 나타났다. 그러나 QYZ지역은 기상 자료와 복사요소가 모두 좋은 일치도를 보였기 때문에, 모형의 모수와 관련된 오차가 주요 원인의 하나로 판단된다. 임관 전도도의 오차가 증발산 오차에 미치는 영향을 분석한 결과, HBG지역을 제외한 다른 연구지 역에서 r값이 0.59~0.82로 관측값과의 상관성이 높은 것을 확인하였다. 또한 MODIS로부터 산출된 순간 증발산을 일 단위로 확장시킨 결과, 순간 증발산의 일치도가 떨어졌던 3개 연구지역을 제외하고 6개 지역에서 <TEX>$r^2$</TEX>가 0.44~0.89, ME와 RMSE는 각각 <TEX>$-0.7{\sim}+0.6mm\;day^{-1}$</TEX>, <TEX>$0.5{\sim}1.1mm\;day^{-1}$</TEX>의 범위로 신뢰도 있는 결과를 나타냈다. Evapotranspiration (ET) is one of the major hydrologic processes in terrestrial ecosystems. A reliable estimation of spatially representavtive ET is necessary for deriving regional water budget, primary productivity of vegetation, and feedbacks of land surface to regional climate. Moderate resolution imaging spectroradiometer (MODIS) provides an opportunity to monitor ET for wide area at daily time scale. In this study, we applied a MODIS-based ET algorithm and tested its reliability for nine flux tower sites in East Asia. This is a stand-alone MODIS algorithm based on the Penman-Monteith equation and uses input data derived from MODIS. Instantaneous ET was estimated and scaled up to daily ET. For six flux sites, the MODIS-derived instantaneous ET showed a good agreement with the measured data (<TEX>$r^2=0.38$</TEX> to 0.73, ME = -44 to <TEX>$+31W\;m^{-2}$</TEX>, RMSE =48 to <TEX>$111W\;m^{-2}$</TEX>). However, for the other three sites, a poor agreement was observed. The predictability of MODIS ET was improved when the up-scaled daily ET was used (<TEX>$r^2\;=\;0.48$</TEX> to 0.89, ME = -0.7 to <TEX>$-0.6\;mm\;day^{-1}$</TEX>, <TEX>$RMSE=\;0.5{\sim}1.1\;mm\;day^{-1}$</TEX>). Errors in the canopy conductance were identified as a primary factor of uncertainty in MODIS-derived ET and hence, a more reliable estimation of canopy conductance is necessary to increase the accuracy of MODIS ET.

Estimating hourly crop ET using a two-source energy balance model and multispectral airborne imagery

Efficient water use through improved irrigation scheduling is expected to moderate fast declining groundwater levels and improve sustainability of the Ogallala Aquifer. An accurate estimation of spatial actual evapotranspiration (ET) is needed for this purpose. Therefore, during 2007, the Bushland ET and Agricultural Remote Sensing Experiment (BEAREX07) was conducted at the USDA-ARS, in Bushland, Texas, to evaluate remote sensing (RS)-based surface energy balance models. Very high-resolution aircraft images were acquired using the Utah State University airborne multispectral system. Instantaneous ET was estimated using a two-source energy balance model (TSM). A minor modification was made in the calculation of sensible heat fluxes to improve ET estimation. Furthermore, a sensitivity analysis of selected variables was conducted to evaluate their effect on ET estimation. Data from four weighing lysimeters, planted to sorghum and corn, were used for evaluating ET predictions. Instantaneous ET was predicted with mean bias error and root mean square error of 0.03 and 0.07 mm h−1 (4.3 and 11.7%), respectively. Results indicated that crop height, roughness length for momentum transfer, clumping factor and soil resistance sub-models need to be refined. Nevertheless, the application of the TSM using high-resolution RS imagery in the Southern High Plains is promising.

Development of a Simple Remote Sensing EvapoTranspiration model (Sim-ReSET): Algorithm and model test

Remote sensing (RS) has been considered as the most promising tool for evapotranspiration (ET) estimations from local, regional to global scales. Many studies have been conducted to estimated ET using RS data, however, most of them are based partially on ground observations. In this study, we developed a new dual-source Simple Remote Sensing EvapoTranspiration model (Sim-ReSET) based only on RS data. One merit of this model is that the calculation of aerodynamic resistance can be avoided by means of a reference dry bare soil and an assumption that wind speed at the upper boundary of atmospheric surface layer is homogenous, but the aerodynamic characters are still considered by means of canopy height. The other merit is that all inputs (net radiation, soil heat flux, canopy height, variables related to land surface temperature) can be potentially obtained from remote sensing data, which allows obtaining regular RS-driven ET product. For the purposes of sensitivity analysis and performance evaluation of the Sim-ReSET model without the effect of potential uncertainties and errors from remote sensing data, the Sim-ReSET model was tested only using intensive ground observations at the Yucheng ecological station in the North China Plain from 2006 to 2008. Results show that the model has a good performance for instantaneous ET estimations with a mean absolute difference (MAD) of 34.27 W/m(2) and a root mean square error (RMSE) of 41.84 W/m(2) under neutral or near-neutral atmospheric conditions. On 12 cloudless days, the MAD of daily ET accumulated from instantaneous estimations is 0.26 mm/day, and the RMSE is 0.30 mm/day. (C) 2009 Elsevier B.V. All rights reserved.

A Review of Current Methodologies for Regional Evapotranspiration Estimation from Remotely Sensed Data

An overview of the commonly applied evapotranspiration (ET) models using remotely sensed data is given to provide insight into the estimation of ET on a regional scale from satellite data. Generally, these models vary greatly in inputs, main assumptions and accuracy of results, etc. Besides the generally used remotely sensed multi-spectral data from visible to thermal infrared bands, most remotely sensed ET models, from simplified equations models to the more complex physically based two-source energy balance models, must rely to a certain degree on ground-based auxiliary measurements in order to derive the turbulent heat fluxes on a regional scale. We discuss the main inputs, assumptions, theories, advantages and drawbacks of each model. Moreover, approaches to the extrapolation of instantaneous ET to the daily values are also briefly presented. In the final part, both associated problems and future trends regarding these remotely sensed ET models were analyzed to objectively show the limitations and promising aspects of the estimation of regional ET based on remotely sensed data and ground-based measurements.

Satellite‐based near‐real‐time estimation of irrigated crop water consumption

Estimation of evapotranspiration using satellite sensors offers the potential for improved water management in irrigated areas. However, previous applications of remote sensing to estimate crop water use have been retrospective in nature, whereas the spatial resolution of sensors that provide information in real time has been thought to be too coarse for such purposes. We describe application of a variation of the Moderate Resolution Imaging Spectroradiometer (MODIS) standard evapotranspiration algorithm for near‐real‐time method for estimation of actual evapotranspiration based entirely on satellite data. The latency of the approach is typically 3 days to 1 week, although the lag could be reduced. The method works best over areas where there is substantial diversity in vegetation types within the remote sensing window; the contrast in vegetation between irrigated and adjacent unirrigated areas meets this requirement well. The satellite sensors/products are MODIS land cover (MOD12Q1), surface reflectance (MOD09GQ), vegetation indices (MOD13Q1), land surface temperature/emissivity (MOD11A1), and albedo (MCD43A3) and NOAA/NESDIS surface radiation budget (SRB) products derived from the Geostationary Operational Environmental Satellites. The MODIS/SRB evapotranspiration estimates agree favorably with ground flux tower observations and evapotranspiration estimates from a much higher resolution Landsat‐based (METRIC) method over irrigated areas of the Klamath River Basin, with instantaneous evapotranspiration biases less than 10% and daily evapotranspiration biases less than 15%. There is a tendency for the MODIS/SRB approach to underestimate, and for the METRIC‐based algorithm to overestimate, seasonal evapotranspiration relative to the tower flux observations.

A Simple Method to Disaggregate Passive Microwave-Based Soil Moisture

This paper develops two alternative approaches for downscaling passive microwave-derived soil moisture. Ground and airborne data collected over the Walnut Gulch experimental watershed during the Monsoon'90 experiment were used to test these approaches. These data consisted of eight micrometeorological stations (METFLUX) and six flights of the L-band Push Broom Microwave Radiometer (PBMR). For each PBMR flight, the 180-m resolution L-band pixels covering the eight METFLUX sites were first aggregated to generate a 500-m ldquocoarse-scalerdquo passive microwave pixel. The coarse-scale-derived soil moisture was then downscaled to the 180-m resolution using two different surface soil moisture indexes (SMIs): (1) the evaporative fraction (EF), which is the ratio of the evapotranspiration to the total energy available at the surface; and (2) the actual EF (AEF), which is defined as the ratio of the actual-to-potential evapotranspiration. It is well known that both SMIs depend on the surface soil moisture. However, they are also influenced by other factors such as vegetation cover, soil type, root-zone soil moisture, and atmospheric conditions. In order to decouple the influence of soil moisture from the other factors, a land surface model was used to account for the heterogeneity of vegetation cover, soil type, and atmospheric conditions. The overall accuracy in the downscaled values was evaluated to 3% (vol.) for EF and 2% (vol.) for AEF under cloud-free conditions. These results illustrate the potential use of satellite-based estimates of instantaneous evapotranspiration on clear-sky days for downscaling the coarse-resolution passive microwave soil moisture.

Uncertainty analysis for satellite derived sensible heat fluxes and scintillometer measurements over Savannah environment and comparison to mesoscale meteorological simulation results

Three methods for estimating instantaneous sensible heat flux ( H) over Savannah environment in West Africa were compared: first, satellite derived estimations using the Surface Energy Balance Algorithm for Land (SEBAL) method [Bastiaanssen, W.G.M., Menenti, M., Feddes, R.A., Holtslag, A.A.M., 1998a. A remote sensing energy balance algorithm for land, SEBAL: 1. Formulation. J. Hydrol. 212–213, 198–212]; secondly, measurements at two test sites in Ghana with a large-aperture scintillometer (LAS); third, high resolution mesoscale meteorological simulations using the MM5 (5th-Generation Penn State/NCAR) mesoscale modelling system. Satellite-derived sensible heat flux was based on seven NOAA-16 AVHRR images that were processed for a 2-week period in December 2001 (dry season) and were compared to LAS-data and MM5 simulation results. A methodology based on Gaussian Error Propagation is presented to derive uncertainties in satellite derived sensible heat flux due to (a) input data, (b) coefficients to determine leaf area index (LAI) and (c) methodological differences in estimating surface temperature T 0. Total computed relative uncertainty in H was 15% for the Tamale test site and 20% for the Ejura site. Uncertainties in instantaneous evapotranspiration λ E, however, are much smaller than uncertainties of H. This results due to the same bias in H and R n − G. For LAS-data, an uncertainty analysis due to input data was performed which showed relative uncertainty of 8% for the Tamale site and 7% for Ejura. Satellite derived net radiation ( R n) was underestimated in comparison to ground measurements which finally caused an underestimation of H. Satellite estimates of H using spatially interpolated ground based measurements of net radiation showed good agreement to LAS data. MM5-computed latent heat flux showed very low values for the entire region. This caused a serious relative MM5-overestimation of sensible heat flux in comparison to LAS and satellite derived estimates. It could be shown that Gaussian Error Propagation can serve as an essential tool to asses the reliability of satellite derived sensible heat fluxes. The resulting uncertainties give information on sensitivities in estimating H and therefore provide a tool for validation purposes.

Retrieval of evapotranspiration over the Alpilles/ReSeDA experimental site using airborne POLDER sensor and a thermal camera

Knowledge of land surface evapotranspiration is of prime interest for environmental applications, such as optimizing irrigation water use, especially in arid and semiarid rangelands where water shortage is a critical problem. Numerous methods aiming at estimating evapotranspiration have been proposed in the literature. In the current paper, we assess the potential of the evaporative fraction based on the S-SEBI (Simplified Surface Energy Balance Index) concept for estimating instantaneous evapotranspiration, and extend this potential to the retrieval of daily evapotranspiration (ET d). To this end, a feasible and operational method is developed, which allows mapping instantaneous and daily evapotranspiration over the Alpilles ReSeDA (Remote Sensing Data Assimilation) experimental area. The method is implemented using data collected with two airborne sensors: PolDER (Polarization and Directionality of Earth Reflectance) and a Thermal camera. The validation over test sites shows that daily evapotranspiration can be obtained within an error of 1 mmd − 1 . Finally a daily evapotranspiration image for the Alpilles ReSeDA experimental site is presented.

Estimating evapotranspiration of European forests from NOAA-imagery at satellite overpass time: Towards an operational processing chain for integrated optical and thermal sensor data products

Evapotranspiration (ET) using the Integral NOAA-imagery processing Chain (iNOAA-Chain) is quantified by implementing visible and thermal satellite information on a regional scale. ET is calculated based on the energy balance closure principle. The combination of evaporative fraction (EF), soil heat flux and instantaneous net radiation, results in an instantaneous spatial distribution of ET values. Surface broadband albedo and land surface temperature (LST) serve to determine EF. EF is derived using four methods based on NOAA/AVHRR satellite imagery. Instantaneous evapotranspiration, i.e. at time of satellite overpass, on European continental scale with emphasis on forest stands is estimated using the iNOAA-Chain. Finally, the estimated net radiation ( R n), soil heat fluxes ( G 0) and evaporative fraction and evapotranspiration at time of satellite overpass are validated against EUROFLUX site data for the growing season of 1997 (March–October). The regression line for the pooled R n (iNOAA-Chain versus EUROFLUX) has a slope, intercept, Pearson product moment correlation coefficient ( R 2) and relative root mean square error (RRMSE) of respectively 0.943, 17.120, 0.926 and 5.5%. The soil heat fluxes, calculated with two approaches are not-well modelled with slopes smaller than − 3.000 and a R 2 in the order of zero. We observe a slight underestimation of the iNOAA Chain estimated EF. The regression line for pooled EF data for the best performing method (SPLIT-method) has a slope of 0.935, an intercept of 0.041 and the R 2 is 0.847. A pooled RRMSE EF value of 12.3% is found. The pooled slope, intercept, R 2 and RRMSE for EF derived with SORT-method 1 are respectively 0.449, 0.251, 0.043 and 65.1%, with SORT-method 2, 0.567, 0.203, 0.174 and 39.1%, and with SORT-method 3, 0.568, 0.254, 0.288, and 32.8%. Also instantaneous evapotranspiration is underestimated with a pooled RRMSE on ET of 23.4%. The regression curve of pooled ET data for the best performing method has a slope of 0.889 an intercept of 15.880 and a Pearson product moment correlation coefficient of 0.771. The other method gives a slope of 0.781, an intercept of 17.541 and a R 2 of 0.776. Error propagation analysis reveals that the relative error on evapotranspiration at satellite overpass time is at least 27%.

PEAR WATER RELATIONS UNDER PARTIAL ROOTZONE DRYING

The effects of partial rootzone drying (PRD) on microjet-irrigated pear trees on a loam soil in the Victorian Goulburn Valley, Australia were investigated during the 2002/03 season. PRD effects on soil water content, midday stem water potential (ψ ψ ψ ψ stem), leaf conductance (gl) and sap velocity were examined. Treatments included 50% (PRD50) and 100% (PRD100) of estimated crop water requirement (ETc) applied daily to one side of the tree on a 14-day alternating cycle while the Control treatment received 100% of ETc applied to both sides of the tree. ETc was calculated using reference crop evapotranspiration (ETo) and FAO crop coefficient for pear adjusted for effective canopy cover (ECC). ECC was measured by canopy PAR interception over the planting square at solar noon and remained at low levels (≤32 %) during the season. ECC did not differ between treatments. Irrigation inputs to harvest were 1.9 Ml ha -1 for PRD50, and 3.5 Ml ha -1 for the fully watered PRD100 and Control trees, compared to the calculated ETc of 3.1 Ml ha -1 over the same period. Crop water relations of the PRD100 trees during a 14-day PRD cycle, as measured by ψ ψ ψ ψ stem and gl, was similar to the Control. Decreased ψ ψ ψ ψ stem under PRD50 indicated water stress. Sap velocity, which was assumed to equate to transpiration, increased in a curvilinear fashion to instantaneous ETo in all trees. This sap velocity-ETo relationship was maintained throughout the 14-day PRD cycle. No evidence of transient changes in stomatal control of transpiration over a drying cycle occurred in the PRD trees. These results indicate that PRD50 led to water stress. Furthermore there was no evidence of the PRD stomatal control and maintenance of plant water status previously reported for grapevines (Dry and Loveys, 1999; Stoll, 2000). Thus, further work on crop species and soil type influence with respect to PRD is required. Also, research into the establishment of ψ stem baselines for fully watered crops would aid irrigation management of orchards.

Evaluation of an enclosed portable chamber to measure crop and pasture actual evapotranspiration at small scale

An enclosed portable chamber was constructed and calibrated to measure actual evapotranspiration (ET) from crop and pasture and then evaluated against established methods that are used to determine evapotranspiration. The chamber was equipped with variable speed electric fans to mix the air within the chamber during each ET measurement. The most appropriate fan speed was investigated. Pasture ET measured using the enclosed portable chamber compared well with predicted water loss using the water balance method for a 6-day period during winter 1997 in Armidale (NSW, Australia). Mean cumulative pasture ET for the 6-day period was 5.8 and 5.9 mm measured with the enclosed portable chamber and water balance method, respectively. Wheat crop ET measured using the enclosed portable chamber was compared with that estimated by the Bowen ratio (BR) method for a 2-day period in the early growth stages of the crop. Mean ET using the enclosed portable chamber was 2.4 mm per day compared with 2.3 mm per day using the BR method. Results from the enclosed portable chamber method showed sensitivity to the choice of fan speed. A slow fan speed that produced an air velocity of 2.7 km/h, gave the closest agreement with the Bowen ratio method (2.3 mm per day versus 2.2 mm per day) for the wheat crop. The main attractions of the enclosed portable chamber method include: (1) its suitability for ET measurement within small areas (<1 m 2), which enables ET measurement from individual plant communities within small areas; (2) its main principles of measuring the actual water flux from transpiring vegetation rather than inferring it from climatic parameters; (3) the speed by which an instantaneous ET rate is obtained (less than 1 min); (4) instantaneous ET measurement can be repeated throughout the day from the same plant communities; and (5) the portability of the enclosed portable chamber. ET measurement using the enclosed portable chamber method may be combined with the existing soil water balance models for comparing alternative crop and pasture systems in terms of their water balance.

Evaluation of daily evapotranspiration estimates from instantaneous measurements

Evapotranspiration on a regional scale is useful information for agricultural and hydrological practice and can be determined by combining remote sensing data with ground-based measurements. However, estimates of evapotranspiration using remote sensing data are usually instantaneous values and to obtain daily evapotranspiration at a regional scale, it is necessary to convert these values into daily totals. By using the data obtained during the HAPEX-MOBILHY experiment in southwestern France, two different methods of converting instantaneous evapotranspiration into daily totals were evaluated. The first method is based on the assumption that the diurnal course of evapotranspiration is similar to that of solar irradiance and can be approximated by a sine function. As a result, daily evapotranspiration can be obtained from one instantaneous measurement. The second method assumes that the evaporative fraction, defined as the ratio of the latent heat flux and the available energy flux, is constant during the daytime period. Hence daily evapotranspiration can be determined from the evaporative fraction and daily total available energy. Comparisons of estimated and measured daily evapotranspiration indicated that both the methods are accurate for cloud-free days. However, the first method is preferable for the purpose of estimating regional evapotranspiration using remote sensing data.

Evaluation of the portable chamber technique for measuring canopy gas exchange by crops

An experiment was conducted to evaluate the portable chamber technique for concurrent measurement of canopy evapotranspiration ( ET) and carbon exchange rate (CER) and to validate the ET measurements using weighing lysimeters. Hourly gas exchange measurements were made on a half-day diurnal basis over a wide range of leaf area indices (0.2–3.8 m 2 m −2) on peanut ( Arachis hypogaea L.) for both irrigated and dry soil conditions. The chamber technique used the LI-COR LI-6200 portable photosynthesis system with an open leaf chamber inside the canopy chamber. Measurement of vapor pressure was achieved with the leaf chamber humidity sensor and thermocouple, while carbon dioxide was cycled through the system infra-red gas analyzer. Full-sun and dark CER measurements were made to compute gross photosynthesis ( PG). Based on replicated measurements, the average coefficients of variation for PG and ET were 15% and 17%, respectively. The field and lysimeter sites gave comparable values of ET and PG. The PG responses to varying light and LAI conditions were consistent with previously cited values. Computed chamber ET must be extrapolated back to the time the chamber is closed to obviate underestimation of the ET rate (1% s −1) owing to the increase in vapor pressure in the chamber. A correction in ET for reduced light owing to radiation transmission losses appears to be unnecessary. Under varying light conditions where measurements were intentionally made during the sunny periods, the chamber overestimated the instantaneous lysimeter ET. Under clear sky conditions, chamber ET compared well with both instantaneous and hourly lysimeter ET. In the latter comparison, errors were less than 0.13 mm h −1 with and R 2 of 0.90. This good agreement was consistent over the morning hours, across the range of leaf area indices, and for both wet and dry soil conditions.