HighlightsThe FAO-56 dual crop coefficient procedure was applied over the entire agricultural areas of Idaho and Nevada to determine evapotranspiration (ET) and net irrigation water requirements (IWR).Basal crop coefficients were expressed as functions of normalized cumulative growing degree days.ET during dormant seasons was included in the estimates.The procedure was applied to a U.S. West-wide study of climate change effects on ET and IWR.Abstract. The FAO-56 dual crop coefficient procedure was used to determine evapotranspiration (ET) and net irrigation water requirements for all agricultural areas of the states of Idaho and Nevada and in a western U.S. study on effects of climate change on future irrigation water requirements. The products of the applications are for use by state governments for water rights management, irrigation system planning and design, wastewater application system design and review, hydrologic water balances, and groundwater modeling. The products have been used by the U.S. federal government for assessing impacts of current and future climate change on irrigation water demands. The procedure was applied to data from more than 200 weather station locations across the state of Idaho, 200 weather station locations across the state of Nevada, and eight major river basins in the western U.S. for available periods of weather records. Estimates were made over daily, monthly, and annual time intervals. Methods from FAO-56 were employed for calculating reference ET and crop coefficients (Kc), with ET calculations performed for all times of the calendar year including winter. Expressing Kc as a function of thermal-time units allowed application across a wide range of local climates and elevations. The ET estimates covered a wide range of agricultural crops grown in the western U.S. plus a number of native plant systems, including wetlands, rangeland, and riparian trees. Evaporation was estimated for three types of open-water surfaces ranging from deep reservoirs to small farm ponds. Keywords: Consumptive use, Dual crop coefficient, Evapotranspiration, FAO-56, Irrigation water requirements.

Evapotranspiration and net irrigation water requirement estimates were updated in 2007 by Allen and Robison (2007; Allen et al., 2007) for agricultural areas in Idaho. ET calculation procedures were employed that use an updated procedure to calculate crop coefficients that considers the impact of surface wetting by irrigation and precipitation on total evapotranspiration. ET was calculated for daily, monthly and annual timesteps for 123 weather station locations across Idaho for complete, available periods of record. ET estimates were made for all times during the calendar year including winter to provide design and operation information for managing land application of agriculture, food processing and other waste streams and to provide full-calendar year estimates of E and ET for hydrologic studies and to estimate beginning of growing season soil water content.

Recent satellite image processing developments have provided the means to calculate evapotranspiration (ET) as a residual of the surface energy balance to produce ET “maps.” These ET maps (i.e., images) provide the means to quantify ET on a field by field basis in terms of both the rate and spatial distribution. The ET images show a progression of ET during the year or growing season as well as its spatial distribution. The mapping evapotranspiration at high resolution with internalized calibration (METRIC) is a satellite-based image-processing procedure for calculating ET. METRIC has been applied with high resolution Landsat images in southern Idaho, southern California, and New Mexico to quantify monthly and seasonal ET for water rights accounting, operation of ground water models, and determination of crop coefficient populations and mean curves for common crops. Comparisons between ET by METRIC, ET measured by lysimeter, and ET predicted using traditional methods have been made on a daily and monthly basis for a variety of crop types and land uses. Error in estimated growing season ET was 4% for irrigated meadow in the Bear River basin of Idaho and 1% for an irrigated sugar beet crop near Kimberly, Id. Standard deviation of error for time periods represented by each satellite image averaged about 13 to 20% in both applications. The results indicate that METRIC and similar methods such as SEBAL hold substantial promise as efficient, accurate, and inexpensive procedures to estimate actual evaporation fluxes from irrigated lands throughout growing seasons.

The present paper describes the development and validation of canopy reflectance-based crop coefficients for potato using canopy reflectance (measured using hand-held radiometers and high resolution multispectral digital imagery), and extensive crop biophysical sampling in selected potato growing fields in and around Kimberly, Idaho, during 1998 and 1999 seasons. Daily crop evapotranspiration was estimated using basal and canopy reflectance-based crop coefficients, and a hydrologic water balance was conducted in the plant root zone. Independent measurements of actual soil moisture measurements were made by neutron probe and gravimetric methods, and used to validate the simulated results. The results validate the reflectance-based crop coefficient method. High resolution multispectral aerial imagery was used to highlight the spatial variability of actual crop water demand in the study fields.

The agricultural use of water is higher than 85% in the western USA, resulting in an increasing water deficit in the region; this situation is commonly encountered throughout the world where irrigated and irrigation-assisted production systems are operational. The objective of this study was to examine differences among dry bean (Phaseolus vulgaris L.) landraces and cultivars in terms of water use efficiency (WUE), subsequently identifying those with a high water use efficiency. Six medium-seeded (25–40 g 100 seed wt−1) landraces and cultivars of pinto and red market classes were evaluated in intermittent drought-stressed (DS) and non-stressed (NS) environments at Kimberly, Idaho, USA in 2003 and 2004. Each market class comprised one each of a landrace and old and new cultivars. Mean evapotranspiration (ET) in the NS environment was 384 mm in 2003 and 432 mm in 2004; the respective ET values in the DS environment were 309 and 268 mm. Mean seed yield was higher in the DS (2678 kg ha−1) and NS (3779 kg ha−1) environments in 2004 than in 2003 (688 and 1800 kg ha−1, respectively). Under severe drought stress in 2003, WUE in the pinto bean ranged from 1.5 for the Common Pinto landrace to 4.4 kg ha−1 mm−1 water for cv. Othello. The Common Red Mexican landrace had the highest WUE (3.7), followed by cvs. NW 63 (2.8) and UI 259 (1.4) in the red market class. Under favorable milder climatic conditions in 2004, the mean WUE value was 10 kg ha−1 mm−1 water in the DS environment and 8.7 kg ha−1 mm−1 water in the NS environment. We conclude that dry bean landraces and cultivars with high WUE should be used to reduce dependence on irrigation water and to develop drought-resistant cultivars to maximize yield and WUE.

A remote sensing approach was applied to estimate near‐noon values of shortwave albedo (α), the fraction of solar radiation reflected by a surface, for alfalfa and tall fescue grass at Kimberly, Idaho. The approach was based on the (P/T) ratio, which is the ratio of the partial radiation (P) sensed by a multi‐band radiometer and the total incident radiation (T) in a given wavelength range. It was found that instead of being constant, as previously suggested, the upward component of the (P/T) ratio under clear‐sky conditions [(P/T)u] followed a logistic growth function of solar altitude angle (Λz) for both crops (r 2 = 0.84). The downward component [(P/T)d], on the other hand, linearly increased with Λz (r 2 = 0.83). By applying the (P/T) ratio methodology, using variable ratios, it was found that the diurnal pattern of clear‐sky α for both crops followed a decreasing function of Λz (r 2 = 0.80). Near‐noon α values for alfalfa estimated using remote sensing were linearly related to plant canopy height (h) (r 2 = 0.92), but not to Λz. For grass, on the other hand, the near‐noon α values obtained by remote sensing were not correlated with either h or Λz. The near‐noon α values for alfalfa obtained with remote sensing deviated considerably from those estimated using an empirical function of day of the year (DOY). For alfalfa, the near‐noon net radiation (R n) values calculated using α values derived by remote sensing were better correlated to measured R n values than those obtained using α estimated as a function of DOY. For grass, the α values derived from remote sensing did not significantly improve the accuracy of the calculated near‐noon R n compared with using α values estimated as a function of Λz.

Continued development of networks of electronic weather stations worldwide has increased the availability of weather data for calculating ETo on an hourly basis. There has been question and debate as well as studies on the appropriate expression and parameterization for the surface resistance (rs) parameter of the Penman-Monteith (PM) equation and the associated coefficient for the reduced form FAO-PM equation when applied hourly. This paper reviews the performance of the FAO-PM method using rs = 70 s m−1 for hourly periods and using a lower rs = 50 s m−1 value during daytime and rs = 200 s m−1 during nighttime. Variability in hour to hour trends in rs among locations and dates makes it difficult, if not impossible, to establish a consistent algorithm for rs. However, the relatively good and consistent accuracy in ETo when using a constant rs = 50 s m−1 during daytime gives good reason to recommend this value as a standardized parameter and coefficient for calculating ETo. Based on a national study in the U.S. and studies by European and American researchers, the authors recommend that the FAO-PM ETo method from FAO56, when applied on an hourly or shorter basis, use rs = 50 s m−1 for daytime and rs = 200 s m−1 for nighttime periods. This use will provide, on average, good agreement with computations made on a 24-h time step basis. No changes are suggested for the FAO-PM method for daily (24-h) time steps, where use of rs = 70 s m−1 should continue.

Sensible heat flux (H) can be an important component of the energy balance of earth surfaces, especially under dry soil conditions and incomplete canopy cover. Estimating H from remote sensing often requires correcting for atmospheric stability to account for the effect of buoyancy, and several simplified methods have been used to make this correction. In this study, data collected from alfalfa and tall fescue grass fields at Kimberly, Idaho, were used to: (1) determine how near‐noon values of stability‐corrected aerodynamic resistance for heat transfer (r ahc) calculated with seven different methods compared with those obtained using a standard micrometeorological method, (2) determine whether the method used to calculate r ahc had a significant effect on near‐noon H calculated using a remote sensing resistance model, and (3) assess the validity of the remote sensing model for determining near‐noon H. It was found that discrepancies in r ahc between methods, compared with the standard method, increased for low wind speeds (u 2) (u 2<2 m s−1), and there was good agreement for five of the seven methods compared for u 2>2 m s−1. Those five methods also had good agreement with the standard method when they were used to calculate near‐noon H for both surfaces. The agreement was good even for low u 2, despite the disagreement in r ahc at low u 2. The other two methods were well correlated with the standard method, but showed significant bias. It was also found that near‐noon H values obtained from remote sensing were well correlated with Bowen ratio measurements but were statistically different from each other for both surfaces.

Drought is a worldwide constraint to dry bean (Phaseolus vulgaris L.) production. The objective of this research was to determine the response of three dry bean landraces and 13 cultivars evaluated under non-stressed (NS) and intermittent drought-stressed (DS) environments at Kimberly, Idaho in 2003 and 2004. The NS received seven irrigations in 2003 and five in 2004, and DS only four in 2003 and two in 2004. Most water use occurred within the top 0.5 m soil in both the NS and DS. Drought reduced biomass and seed yield, harvest index, and seed weight. Maturity was delayed in severe drought, but was similar or shortened by 1 to 6 d under moderate drought. Mean seed yield was reduced by 62% in 2003 and by 27% in 2004. Common Red Mexican and CO 46348 had high seed yield in both NS and DS environments, whereas 'Matterhorn' and 'Othello' yielded comparatively high under DS but moderately in NS environment. Drought resistance was inadvertently reduced from Common Red Mexican landrace to intermediate levels in 'NW-63' and 'UI 239' released in 1979 and 1993, respectively, and more recently released 'LeBaron' (1999) and 'UI259' (1996) were susceptible. Conversely, drought resistance was increased in newer pinto (Othello 1986; CO 46348) and great northern (Matterhorn 1998) releases compared to the landraces and older cultivars tested for those market classes. Seed yield in NS and DS was positively correlated. Seed yield was also correlated with harvest index in DS and NS. All early maturing cultivars except Othello (e.g., UI 59, US 1140, Common Pinto, Topaz, UI 320, and LeBaron) were susceptible. Common Red Mexican did not have any reduction in seed weight due to drought stress. Drought resistant genotypes should be used for determining irrigation frequency, amount of water to be applied, and mechanisms of resistance and for identifying, mapping, and pyramiding favorable genes for dryland and irrigation-assisted sustainable production systems.

This paper offers a historical retrospective on the remote sensing of crop coefficients for obtaining actual crop evapotranspiration. We present the canopy reflectance-based approach of crop coefficients and show the usefulness of high-resolution airborne imagery as a tool for monitoring the actual crop growth changes and characterizing in-field variability in an objective manner.

Soil heat flux (G) is an important component of evapotranspiration (ET) modeling, especially for estimating ET values for hourly or shorter periods. In this study, meteorological and agronomic measurements were made at Kimberly, Idaho, with the purpose of establishing empirical relationships to estimate G for alfalfa and clipped tall fescue grass. For both plant surfaces, good linear correlation was found for most days between the averages of the 20-min net radiation (Rn) and G values for a given day. However, when the soil surface was wet, after rain or irrigation, the relationship was subject to hysteresis problems. The linear relationship between G and Rn for alfalfa also changed with plant canopy height (h), and an equation was derived to estimate G from Rn and h (r2 = 0.88). This equation fitted measured G data much better than two other commonly used models (Allen et al., 1996; Clothier et al., 1986). For tall fescue grass, h did not affect the relationship between Rn and G, as the grass was clipped weekly resulting in a narrow range of h (0.09 to 0.19 m). A linear equation to estimate G as a function of Rn (r2 = 0.91) was derived for clipped tall fescue grass, which was found to fit measured data equally well as the model proposed by Allen et al. (1998), but that uses a single equation for both daytime and nighttime instead of two separate equations.

Quantifying evapotranspiration (ET) from agricultural fields is important for field water management. water resources planning, and water regulation. Traditionally, ET from agricultural fields has been estimated by multiplying the weather-based reference ET by crop coefficients (K-c) determined according to the crop type and the crop growth stage. Recent development of satellite remote sensing ET models has enabled us to estimate ET and K-c for large populations of fields. This study evaluated the distribution of K, over space and time for a large number of individual fields by crop type using ET maps created by a satellite based energy balance (EB) model. Variation of K-c curves was found to be substantially larger than that for the normalized difference vegetation index because of the impacts of random wetting events on K-c especially during initial and development growth stages. Two traditional K-c curves that are widely used in Idaho for crop management and water rights regulation were compared against the satellite-derived K, curves. Simple adjustment of the traditional K-c curves by shifting dates for emergence, effective full cover, and termination enabled the traditional curves to better fit K-c curves as determined by the EB model. Applicability of the presented techniques in humid regions having higher chances of cloudy dates was discussed.

SEBAL and METRIC remote sensing energy-balance based evapotranspiration (ET) models have been applied in the western United States. ET predicted by the models was compared to lysimeter-measured ET in agricultural settings. The ET comparison studies showed that the ET estimated by the remote sensing models corresponded well with lysimeter-measured ET for agricultural crops in the semi-arid climates. Sensitivity analyses on impacts of atmospheric correction for surface temperature and albedo showed that the internal calibration procedures incorporated in the models helped compensate for errors in temperature and albedo estimation. A repeatability test by two totally independent model applications using different images, operators and weather datasets showed that seasonal estimations by the models have high repeatability (i.e. stable results over ranges in satellite image timing, operator preferences and weather datasets). These results imply that the SEBAL/METRIC remote sensing models have a high potential for successful ET estimation in the semi-arid United States.