Evapotranspiration Balance Research Articles

Satellite-based water use dynamics using historical Landsat data (1984–2014) in the southwestern United States

Remote sensing-based field-scale evapotranspiration (ET) maps are useful for characterizing water use patterns and assessing crop performance. The relative impact of climate variability and water management decisions are better studied and quantified using historical data that are derived using a set of consistent datasets and methodology. Historical (1984–2014) Landsat-based ET maps were generated for major irrigation districts in California, i.e., Palo Verde and eight other sub-basins in parts of the middle and lower Central Valley. A total of 3396 Landsat images were processed using the Operational Simplified Surface Energy Balance (SSEBop) model that integrates weather and remotely sensed images to estimate monthly and annual ET within the study sites over the 31years. Model output evaluation and validation using gridded-flux data and water balance ET approaches indicated relatively good correspondence (R2 up to 0.88, root mean square error as low as 14mm/month) between SSEBop ET and validation datasets. In a pairwise comparison, annual variability of agro-hydrologic parameters of actual evapotranspiration (ETa), land surface temperature (Ts), and runoff (Q) were found to be more variable than their corresponding climatic counterparts of atmospheric water demand (ETo), air temperature (Ta), and precipitation (P), revealing process differences between regional climatic drivers and localized agro-hydrologic responses. However, only Ta showed a consistent increase (up to 1.2K) over study sites during the 31years, whereas other climate variables such as ETo and P showed a generally neutral trend. This study demonstrates a useful application of “Big Data” science where large volumes of historical Landsat and weather datasets were used to quantify and understand the relative importance of water management and climate variability in crop water use dynamics in regards to the linkages among water management decisions, hydrologic processes and economic transactions. Irrigation district-wide ETa estimates were used to compute historical crop water use volumes and monetary equivalents of water savings for the Palo Verde Irrigation District (PVID). During the peak crop fallowing year in PVID, the water saved reached a maximum of ~107,200acre-feet in 2011 with an estimated monetary payout value of $20.5 million. A significant decreasing trend in actual ET despite an increasing atmospheric demand in PVID highlights the role of management decisions in affecting local hydrologic processes. This study has importance for planning water resource allocation, managing water rights, sustaining agricultural production, and quantifying impacts of climate and land use/land cover changes on water resources. With increased computational efficiency, similar studies can be conducted in other parts of the world to help policy and decision makers understand and quantify various aspects of water resources management.

Differences in sap flux‐based stand transpiration between upper and lower slope positions in a Japanese cypress plantation watershed

AbstractThis study investigated the difference in stand transpiration (E) between upper (UP) and lower (LP) slope plots of a Japanese cypress (Chamaecyparis obtusa) watershed to obtain catchment‐scale E estimates. Sap flux‐based stand‐scale E estimates were conducted on the UP and LP, which had contrasting stand structures (diameter at breast height and tree height) despite being the same age. The plant area index was similar in the UP and LP, while the total sapwood area (AS_stand) and the mean sap flux density (JS) in the UP were 23% and ~30% lower, respectively, than those of the LP. Stand‐scale E is the product of the AS_stand and JS in a stand. As a result, E in the UP was ~50% lower than that in the LP. This ratio was relatively constant throughout the year, which was supported by a similar decline in E, which resulted from soil water decline in the UP and LP. Canopy conductance (GC) in the UP was ~50% smaller than that of the LP. In contrast to previous studies that showed a consistent JS along a slope, the differences in E between the UP and LP were caused by differences in AS_stand and JS, probably because the UP exhibited a lower GC per unit leaf area than the LP. Additionally, evapotranspiration (ET), the sum of sap flux‐scaled E plus rainfall interception, was 8–14% lower than the catchment water balance ET (precipitation minus runoff), which suggests the feasibility of calculating sap flux‐scaled E for the two slope positions. Copyright © 2015 John Wiley & Sons, Ltd.

An evapotranspiration product for arid regions based on the three-temperature model and thermal remote sensing

An accurate estimation of evapotranspiration (ET) is crucial to better understand the water budget and improve related studies. Satellite remote sensing provides an unprecedented opportunity to map the spatiotemporal distribution of ET. However, ET values from barren or sparsely vegetated areas in arid regions are often assumed to be zero in typical ET products because of their low values. In addition, separating ET into soil evaporation (E-s) and vegetation transpiration (E-c) is difficult. To address these challenges, we developed an ET product (MOD3T) based on a three-temperature model and thermal remote sensing, specifically Moderate Resolution Imaging Spectroradiometer (MODIS) data. MOD3T has a spatial resolution of 1 km and a temporal resolution of 8 days. All input parameters except air temperature were obtained from MODIS datasets. Validation in two adjacent arid river basins in northwestern China showed that the mean absolute errors (mean absolute percent errors) between the MOD3T and flux tower ET were 0.71 mm d(-1) (18.5%) and 0.16 mm d(-1) (24.9%) for a densely vegetated area and sparsely vegetated sandy desert, respectively. The error between the MOD3T and water balance ET was 24 mm y(-1) (8.1%). The E-c/ET or E-s/ET of MOD3T was comparable to the observed stable oxygen and hydrogen isotopes. Unlike the MODIS ET (MOD16), MOD3T could not provide continuous ET values (as 70% of the MOD16 area lacked data) but exhibited relatively low uncertainty, particularly in cold seasons. Therefore, MOD3T can provide ET, E-s and E-c estimates for arid regions within acceptable ranges. (C) 2015 Elsevier B.V. All rights reserved.

A simple Landsat–MODIS fusion approach for monitoring seasonal evapotranspiration at 30 m spatial resolution

Persistent cloud-cover in the humid southeastern USA and the low temporal resolution of Landsat sensors limit the derivation of seasonal evapotranspiration (ET) maps at moderate spatial resolution. This article introduces a Landsat Moderate Resolution Imaging Spectroradiometer (Landsat–MODIS) ET fusion model that uses simple linear regression to integrate Landsat-derived reference ET fraction (ETrF) from mapping ET at high resolution with internalized calibration (METRIC) model and the vegetation temperature condition index (VTCI) derived from MODIS images. For a study site in Florida, model-estimated ET and ET estimated using energy budget eddy covariance at a US Geological Survey (USGS) station in Ferris Farm, Florida, were found to be in a good agreement with a root mean squared error of 0.44 mm day–1, coefficient of determination (R2) of 0.80, Nash–Sutcliffe efficiency of 0.79 for daily ET (ETd), and 2% relative error for cumulative seasonal ET during the growing season of 2001. At another study site in Alabama, the model underestimated 2008 annual water balance ET for the Fish River Watershed by 39 mm or 4%. Comparisons of model-estimated ET with that obtained using a non-fusion Landsat-only approach at both sites indicated that the fusion of Landsat and MODIS ET values reduces potential errors in ET estimation that would otherwise arise due to insufficient availability of cloud-free Landsat images for METRIC processing. Validation results and application of the model in deriving seasonal/annual ET for different land-cover classes in the Fish River Watershed suggested that the fusion model has the potential to be used in continuously monitoring ET for field- to watershed-level agricultural and hydrological applications in the southeastern USA.

Approximations of stand water use versus evapotranspiration from three mangrove forests in southwest Florida, USA

Leaves from mangrove forests are often considered efficient in the use of water during photosynthesis, but less is known about whole-tree and stand-level water use strategies. Are mangrove forests as conservative in water use as experimental studies on seedlings imply? Here, we apply a simple model to estimate stand water use (S), determine the contribution of S to evapotranspiration (ET), and approximate the distribution of S versus ET over annual cycles for three mangrove forests in southwest Florida, USA. The value of S ranged from 350 to 511mmyear−1 for two mangrove forests in Rookery Bay to 872mmyear−1 for a mangrove forest along the Shark River in Everglades National Park. This represents 34–49% of ET for Rookery Bay mangroves, a rather conservative rate of S, and 63–66% of ET for the Shark River mangroves, a less conservative rate of S. However, variability in estimates of S in mangroves is high enough to require additional study on the spatial changes related to forest structural shifts, different tidal regimes, and variable site-specific salinity concentrations in multiple mangrove forests before a true account of water use conservation strategies can be understood at the landscape scale. Evidence does suggest that large, well-developed mangrove forests have the potential to contribute considerably to the ET balance; however, regionally most mangrove forests are much smaller in stature in Florida and likely contribute less to regional water losses through stand-level transpiration.

The Holocene environmental changes in boreal fen peatland of northern Mongolia reconstructed from diatom assemblages

The Holocene hydrological changes of fen peatland in northern Mongolia were reconstructed using diatom fossils with an aid of Sphagnum leaf fossils. Analyses were conducted on three cores taken across the basin and reconstructed water environments were compared to find basin-wide hydrological changes and climate anomalies. The peatland has been covered by sedges during most of its 10 thousand years history, but all cores showed common peatland hydrosere processes from fluvial environment in the early Holocene period, shallow marsh to present acidic fen peatland. The establishment of fen peatland at 6.8–6.4 cal ka BP was marked synchronously by the shift of diatom and Sphagnum taxa assemblages and this period was inferred as the onset of the mid-Holocene dry climate which is widely observed around Mongolia. The dry environment lasted until around 2.8 cal ka BP with a temporal wetter condition at 4.4–3.5 cal ka BP. Short term diatom trend shifts indicated temporal wetter environments at 8.7–8.4, 8.0–7.6, 2.4–2.1, 1.2–0.5 cal ka BP and a dry environment at 0.4–0.2 cal ka BP. These periods of hydrology changes were correlative with other studies in Mongolia and East Siberia, so they would represent climate-induced hydrological changes, and those regions might share the same response to long term insolation changes or global climate anomalies. However, disagreements of climate data even within northern Mongolian region suggest large influence of regional geography and the balance of evapotranspiration on climate behaviors as shown by recent dendrological paleoenvironment researches. Coherent differences of main taxa compositions between three cores were attributed to core site-specific local hydrology mainly in terms of relative wetness from wettest core site accompanied by abundant planktonic diatom taxa to driest site with acidic peat indicator taxa. Timings and manners of temporal diatom trend shifts were also largely different between cores suggesting significant difference in each core site sensitivity to basin-wide hydrology changes, confirming the necessity of using multiple cores and proxies in peatland sediments to detect climate-induced environmental changes.

A comprehensive evaluation of two MODIS evapotranspiration products over the conterminous United States: Using point and gridded FLUXNET and water balance ET

Remote sensing datasets are increasingly being used to provide spatially explicit large scale evapotranspiration (ET) estimates. Extensive evaluation of such large scale estimates is necessary before they can be used in various applications. In this study, two monthly MODIS 1km ET products, MODIS global ET (MOD16) and Operational Simplified Surface Energy Balance (SSEBop) ET, are validated over the conterminous United States at both point and basin scales. Point scale validation was performed using eddy covariance FLUXNET ET (FLET) data (2001–2007) aggregated by year, land cover, elevation and climate zone. Basin scale validation was performed using annual gridded FLUXNET ET (GFET) and annual basin water balance ET (WBET) data aggregated by various hydrologic unit code (HUC) levels. Point scale validation using monthly data aggregated by years revealed that the MOD16 ET and SSEBop ET products showed overall comparable annual accuracies. For most land cover types, both ET products showed comparable results. However, SSEBop showed higher performance for Grassland and Forest classes; MOD16 showed improved performance in the Woody Savanna class. Accuracy of both the ET products was also found to be comparable over different climate zones. However, SSEBop data showed higher skill score across the climate zones covering the western United States. Validation results at different HUC levels over 2000–2011 using GFET as a reference indicate higher accuracies for MOD16 ET data. MOD16, SSEBop and GFET data were validated against WBET (2000–2009), and results indicate that both MOD16 and SSEBop ET matched the accuracies of the global GFET dataset at different HUC levels. Our results indicate that both MODIS ET products effectively reproduced basin scale ET response (up to 25% uncertainty) compared to CONUS-wide point-based ET response (up to 50–60% uncertainty) illustrating the reliability of MODIS ET products for basin-scale ET estimation. Results from this research would guide the additional parameter refinement required for the MOD16 and SSEBop algorithms in order to further improve their accuracy and performance for agro-hydrologic applications.

Validation of remotely sensed evapotranspiration over the Hai River Basin, China

Ground‐based validation is crucial for ensuring the accuracy of remotely sensed evapotranspiration (RS_ET) and extending its application. This paper proposes an innovative validation method based on multisource evapotranspiration (ET) from ground measurements, with the validation results including the accuracy assessment, error source analysis, and uncertainty analysis of the validation process. It is a potentially useful approach to evaluate the accuracy and analyze the spatiotemporal properties of RS_ET at both the basin and local scales, and is appropriate to validate RS_ET in diverse resolutions at different time‐scales. An independent RS_ET validation using such a method was presented over the Hai River Basin in 2002–2009, China. In general, validation at the basin scale showed good agreements between the 1 km annual RS_ET and the validation data such as the water balance ET (root‐mean‐square error (RMSE): 50.73 mm), MODIS ET products (RMSE: 79.84 mm), precipitation, and land use types. At the local scale, multiscale ET measurements from large aperture scintillometer (LAS) and eddy covariance system (EC) with a footprint model were used for validation over three typical landscapes. In most cases, the 1 km RS_ET resulted in slight overestimation with the LAS measurements (RMSE: 10.75 mm for monthly results, 0.78 mm for daily results), while the 30 m RS_ET was underestimated compared to the EC measurements (RMSE: 16.28 mm for monthly results, 0.99 mm for daily results). Furthermore, error sources of RS_ET and uncertainties of the validation process were investigated in detail. The results showed that the proposed validation method was reasonable and feasible.

Carbon and water exchange in semiarid ecosystems in SE Spain

The changing climate is affecting biological and physical processes occurring in the biosphere, including the biogeochemical cycles of water and carbon. The response of ecosystem carbon exchange to changes in temperature and the water balance is still very uncertain, a fact that highlights the need for research in order to understand the role of the biosphere in the future global carbon budget. South east Spain is on the borderline between the tropical and mid-latitude climate zones, with climates ranging from sub-humid to arid, and the majority representing the driest area in Europe. Initial predictions on climate change for this region point to decreases in total precipitation and the number of precipitation events, meaning a decrease in water availability underlining the vulnerability of the region to desertification. This region, due to these special climatic conditions, has been the subject of experimental carbon and water field research in recent years. This paper defines the state of the art of carbon and water balance measurement and modelling studies in this region, analyses the different processes involved in aggregate exchanges (vegetation and soil; net ecosystem carbon balance–evapotranspiration, photosynthesis–transpiration, soil respiration–evaporation), and identifies needs for future research.

Using a Surface Energy Balance Model to Calculate Spatially Distributed Actual Evapotranspiration

Remote sensing algorithms are currently being used to estimate regional surface energy fluxes [e.g., latent heat flux or evapotranspiration (ET)]. Many of these surface energy balance models use information derived from satellite imagery such as Landsat, AVHRR, ASTER, and MODIS to estimate ET. The remote sensing approach to estimate ET provides advantages over traditional methods. One of the most important advantages is that it can provide regional estimates of actual ET at low cost. Most conventional methods are based on point measurements (e.g., soil water sensors, lysimeters, and weather station data), limiting their ability to capture the spatial variability of ET. Another advantage of remote sensing/surface energy balance ET models is that they are able to estimate the actual crop ET as a residual of the energy balance without the need of using reference crop ET and tabulated crop coefficients. This paper focuses on the application of the energy balance-based model “Remote Sensing of ET” (ReSET) that...

Developmentevaluation of an actual evapotranspiration estimation algorithm using satellite remote sensingmeteorological observational network in Oklahoma

Actual evapotranspiration (AET) estimation using satellite sensors can improve water resources planningwater regulation in irrigated areas. This paper evaluates an AET estimation algorithm developed by integrating satellite remote sensingan environmental monitoring network in Oklahoma, USA for operational daily water management purpose. First, a surface energy balance evapotranspiration (ET) algorithm is implemented to estimate actual ET by integrating the twice-daily overpass of Moderate Resolution Imaging Spectroradiometer (MODIS) sensor dataOklahoma's world-class environmental monitoring network—Mesonet—with 5-minute data acquisition in real time. Second, accuracy of the estimated ET is evaluated at the site scale using Ameriflux tower latent heat fluxMesonet site crop ET on daily, 8-dayseasonal basis. The results showed that MODIS/Mesonet-AET (MM-AET) estimation showed agreement with ground observations, with daily ET bias less than 15%seasonal bias less than 8%. Additionally, actual ET modelled from a water balance budget analysis in a heavily instrumented basin compares favourably (bias < 3%) with the MM-AET at catchment scales with an order of several hundreds square kilometres. This study demonstrates that (1) the MM-AET estimation is acceptable for daily actual ET estimation(2) it is feasible to implement the proposed MM-AET algorithm in real time for irrigational water resources management at the scale of irrigation projects in Oklahoma.

Vegetation changes and their climatic implication for the late Pleistocene at Lake Poukawa, Hawkes Bay, New Zealand

A detailed vegetation history extending back to the Last Interglacial (Marine Isotope Substage 5e) is presented for Lake Poukawa, Hawkes Bay, New Zealand. This history is based on palynological analyses of a 198-m core record, age secured by uranium/thorium (U/Th) and optically stimulated luminescence (OSL) dates as well as tephrochronology. Vegetation of the penultimate glacial (Isotope Stage 6) probably consisted of a very sparse shrubby herbland of stunted podocarps ( Phyllocladus sp.), daisies, grasses and sedges. Similar floras existed in each of the succeeding cool periods. Four interglacial/interstadial floras are preserved. The peak of the Last Interglacial (Substage 5e) was dominated by extensive lowland forest of tall forest podocarps ( Podocarpus/Prumnopitys), most probably matai ( Prumnopitys taxifolia) with red beech ( Nothofagus fusca), secondary forest trees ( Coprosma spp., Myrsine spp.) and tree ferns ( Cyathea spp.). In the Poukawa basin itself, a swamp forest of the podocarp ( Dacrycarpus dacrydioides) with the true palm ( Rhopalostylis sapida) expanded. Substage 5e was significantly warmer and/or moister than the present day. Substage 5c/5a was marked by montane forest of red and silver beeches ( N. fusca and N. menziesii), Phyllocladus spp. and secondary trees. A reduced representation of Podocarpus/ Prumnopitys possibly consists of montane podocarp ( Podocarpus hallii). We infer the temperature of Substage 5c/5a to have been 3.5–5 °C lower than the present day. Although Stage 3 flora are superficially similar to those of Substage 5c/5a, the beeches are relatively less dominant. The regional flora during Stage 1 is very similar to those of Substage 5e, but the density is lower with no swamp forest during the present interglaciation. This may represent a change in evapotranspiration balance around Poukawa, but may also be controlled by hydrological conditions within the basin.

Field evaluation of passive capillary samplers for estimating groundwater recharge

Passive capillary samplers (PCAPS), which sample water from the vadose zone via a hanging water column in a fiberglass wick, have shown potential to provide superior estimates of soil water flux compared to alternative methods. The objectives of this study were to evaluate the performance of PCAPS under natural rain‐fed conditions concerning (1) their operational characteristics and (2) their ability to estimate soil water flux. Forty‐two PCAPS were installed in 21 commercial agricultural fields in Lane County, Oregon. Monthly measurements of soil water flux and precipitation were recorded at each site for the 4‐year project duration. Of the 42 installed PCAPS, 12 samplers at six sites were inoperable or did not operate efficiently: 10 samplers were consistently below the water table, which overflowed the collection vessels rendering the samplers inoperable. Only two of the PCAPS exhibited technical failure resulting in unusually low collection efficiencies, thought to be due to a collapse of the collection vessel from oversuction during sample retrieval. On average, the 30 remaining PCAPS measured soil water flux 25% greater than that obtained from a water balance estimate. This discrepancy represents ∼8% of the total annual precipitation and irrigation each site received. PCAPS collection efficiency was found to be significantly correlated (R2 = 0.75) to the water balance yearly estimated recharge. The difference between PCAPS measured and water balance estimated percolation could be the result of inaccuracy in water balance evapotranspiration estimates and/or oversampling in the presence of high water tables. To estimate the mean yearly recharge at each site with a 30% bound on the mean at the 0.05 confidence level, eight PCAPS are required. This number corresponds closely to the results of Brandi‐Dohrn et al. [1996a] and is thought to be due to intrinsic variability of percolation.

Assessment of evapotranspiration models applied to a watershed of Canadian Prairies with mixed land-uses

Using meteorological data collected in the summer of 1996 and 1997, three evapotranspiration (ET) models, the Penman–Monteith (PM), the modified Penman for non-saturated surface, and the two-source models were applied to estimate hourly ET from different land-use covers of the Paddle River Basin (area = 265 km2). By assuming closed canopy conditions, the PM model could estimate the ET of coniferous forest and agricultural land because the contribution of soil evaporation under these land covers is not significant. For mixed forest and pasturelands that are partially vegetated, however, the amount of soil evaporation is significant and so PM underestimated the total ET. The modified Penman model mainly underestimated hourly ET during daytime when the atmosphere is unstable, but overestimated ET during early morning and late afternoon (stable atmosphere), particularly for pastureland. By re-establishing a relationship for the relative evaporation, the ET simulated by the modified Penman model improved. Both PM and the modified Penman are assessed with respect to the comprehensive, two-source model's simulated ET because the latter agrees favourably with the ET obtained from the basin water balance. The water balance ET is reliable because it is based on the streamflow, surface temperature, net radiation and soil moisture simulated by the semi-distributed hydrological model, Semi-distributed Physically based Hydrologic Model–Remote Sensing, DPHM-RS (host to the two-source model), all of which have been demonstrated to agree well with the observed data. Copyright © 2000 John Wiley & Sons, Ltd.

Feedbacks in the Land-Surface and Mixed-Layer Energy Budgets

A mixed-layer model of the surface energy budget and the planetary boundary layer (PBL) is developed, based on the prognostic equations for soil temperature, mixed layer potential temperature and specific humidity and the growth and abrupt collapse of the PBL. Detailed parameterizations of the longwave radiative fluxes are included. The feedbacks in the uncoupled (i.e. surface energy budget with non-responding PBL) and coupled land surface and atmospheric mixed-layer energy budgets are examined. A simplified, time continuous, version of the model, in which the specific humidity budget is the balance of evapotranspiration and dry-air entrainment, and the PBL height is given by the lifted condensation level, is shown to be in good agreement with the complete model. By forcing the simplified model with daily mean rather than periodic solar radiation, an equilibrium model state is achieved where the fluxes are in close agreement with the daily mean fluxes corresponding to the periodic forcing. The model also agrees favorably with measurements from the FIFE field experiment. Feedbacks are examined using the equilibrium model state. The uncoupled and coupled model sensitivities with respect to the minimal stomatal resistance and the atmospheric specific humidity not only differ in magnitude, but in sign as well. This results puts into question the extent to which uncoupled land-surface models that are forced with atmospheric variables may be used in sensitivity studies.

Sap flow measurements of transpiration from cotton grown under ambient and enriched CO2 concentrations

Increasing atmospheric CO2 concentration has many implications for agriculture and forestry, one of which is the effect it will have on transpiration (T). The objective of this work was to quantify T of cotton (Gossypium hirsutum L.) grown in the field under ambient (370 μmol mol−1) and enriched (550 μmol mol−1) CO2 concentrations. Measurements were made in 1990 and 1991 at the Maricopa Agricultural Center, Arizona. Constant-power sap flow gauges were used to measure T. In 1990, three plants and in 1991, 10 plants were simultaneously instrumented with gauges in each of the CO2 treatments. Leaf area of plants with gauges was measured. T measured by sap flow was compared with evapotranspiration (ET) calculated by water balance in 1990 and with T calculated by water balance in 1991. Soil evaporation was measured using microlysimeters in 1991, and was found to be essentially equal (approximately 0.8 mm day−1, or about 10% of T) in the two CO2 treatments. There were no consistent differences in leaf area of plants with gauges between the two CO2 treatments. Sap flow, for periods from 15 min to 2 weeks, was not significantly different between the two CO2 treatments in either year, except for a few days in 1990. In 1991, the coefficient of variation of daily sap flow across plants was the same (about 30%) for both CO2 treatments throughout the year. The water balance ET (1990) and T (1991) were similar to sap flow in both years, and also showed no effect of CO2 treatment. These results show that for this crop, grown under well-watered and high-fertility conditions, there was no effect of CO2 on T, on a per unit ground area or per plant basis. These results are relevant for assessing the effects of increasing atmospheric CO2 concentrations on transpiration by cotton.

Crop Coefficients for Estimating Evapotranspiration of Spring Wheat

Four sets of weekly crop coefficients were derived for spring wheat using the water balance evapotranspiration (Det) of two lysimeters in 1985 and 1986, and reference evapotranspiration (ETo) based on the Class A pan model. Cokriging, using the two variables sum of Det and sum of ETo, smoothed the obvious errors in weeks 4 and 5 by estimating substitute values based on serial covariance. Serial autocorrelation in four sets of weekly crop coefficients (Kcrop) was used to forecast with unbiased error a series of 16 Kcrop-values which are recommended for use in irrigation scheduling when ETo is modeled and Det estimated.

Remotely sensed canopy temperature based evapotranspiration models for wheat

Field studies were conducted on differentially irrigated wheat (Cv. ‘Sonalika’), on a sandy loam soil at Indian Agricultural Research Institute, New Delhi to evaluate the surface energy balance evapotranspiration (ET) models which use remotely sensed canopy temperature as an input. Four energy balance ET models were used in this study viz. Jackson et. al. (1977) approach; Bartholic et. al. (1970) and stability corrected and uncorrected aerodynamic resistance forms of models. To test the model, ET was estimated using remotely sensed canopy temperatrue, with the remaining to ET measured by water depletion method. Aerodynamic resistance forms of ET models performed well as compared to other models. Results indicate that remotely sensed canopy temperature as an input to surface energy balance models, offers a potential method of estimating ET from a cropped surface.

Salt and Water Balance, Coachella Valley, California

The salt balance index (output/input) for the irrigated land of Coachella Valley during 1957-1965 was highly related to both the area of irrigated land having tile drainage and the leaching percentage, the index becoming close to one when about half the land became tiled and when the leaching percentage increased to about 30. At salt balance evapotranspiration, taken as the difference between depths of applied and drainage water, agreed within about 10% of that calculated by the Blaney-Criddle formula. About 10% of the applied salt appears to precipitate in the soil largely as calcium carbonate, and calcium and magnesium in water replaces exchangeable sodium in soil during irrigation operations.