James L. Wright

COMPARISON OF ELEVEN VEGETATION INDICES FOR ESTIMATING PLANT HEIGHT OF ALFALFA AND GRASS

A great variety of vegetation indices, derived from remote sensing measurements, are commonly used to characterize the growth pattern of cropped surfaces. In this study, multispectral canopy reflectance data were obtained from grass (Festuca arundinacea) and alfalfa (Medicago sativa L.) at Kimberly, Idaho, with the purpose of comparing the performance of 11 vegetation indices for estimating plant height of these two structurally different crop canopies. An additional purpose was to develop quantitative relationships between plant height and the different vegetation indices, which could be used to estimate plant height from remote sensing inputs. For alfalfa, good logistic growth relationships between plant height and all the different vegetation indices were found. The relationship resulted in r2 > 0.90 for all the vegetation indices, and r2 > 0.97 for most of them. While all the vegetation indices were very sensitive to changes in plant height at the beginning of the growing cycle, the Normalized Difference Vegetation Index (NDVI), the Infrared Percentage Vegetation Index (IPVI), and the Transformed Vegetation Index (TVI) became insensitive to additional plant growth when alfalfa reached heights of 0.45, 0.40, and 0.45 m, respectively. All the other vegetation indices performed reasonably well for the entire range of alfalfa plant heights considered in this study ( 2 ~ 0.76).

GUIDELINES FOR VALIDATING BOWEN RATIO DATA

For a variety of reasons, the measurement of latent heat flux using the Bowen ratio method can sometimes result in erroneous data. This study provides guidelines for detecting erroneous Bowen ratio data and illustrates the application of these guidelines by comparing Bowen ratio and lysimeter data collected over grass and alfalfa in southern Idaho. Errors in net radiation were detected by comparing measured with theoretical values. However, it was found that good theoretical procedures to validate soil heat flux data are lacking. Only empirical equations mainly used for remote sensing applications to obtain estimates close to noontime are available. Extremely inaccurate latent heat fluxes were easily filtered out by rejecting data when the calculated Bowen ratio (β) values were close to -1. A simplified procedure was proposed to reject fluxes with the wrong sign, and three different equations were used successfully to detect the occurrence of condensation inside the type of measurement system used in the study. Guidelines to assure adequate fetch are provided. Fetch did not affect the measured fluxes in this study, which may have been due to the similarity in surface properties between the crops under study and those in the surrounding fields.

Changes in Climate and Estimated Evaporation Across a Large Irrigated Area in Idaho

G round level climatic measurements were taken along a 50 km transect going from dry sagebrush land into the center of a large irrigated area in southern Idaho. Measurements in May, when the desert area was dry, indicated that climatic changes across the transect were minimal. In August, when the desert was obviously very dry, air temperatures decreased, vapor pressure increased, and windspeed was reduced about 40 percent within the irrigated area. The results demonstrate that any weather service agency or group must consider the distance from dry surroundings when selecting sites that are to be representative of climatic conditions over irrigated fields.

ESTIMATING SOIL HEAT FLUX FOR ALFALFA AND CLIPPED TALL FESCUE GRASS

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.

Chapter 8. Water Requirements

Evapotranspiration (ET) calculation guidelines are based on the crop coefficient-reference evapotranspiration method (Kc ETref). Equations for the ASCE-EWRI standardized Penman-Monteith method are provided for grass and alfalfa references, where the grass reference standardization follows the FAO Penman-Monteith procedure. Linearized FAO-style crop coefficients from FAO-56 and curvilinear coefficients from Wright are presented as both mean and as dual (basal) crop coefficients. ET coefficients for landscape utilize a decoupled procedure similar to that summarized by the Irrigation Association Water Management Committee. Guidelines for calculating irrigation water requirements and peak system design rates are described.

NON-WATER-STRESSED BASELINES FOR CALCULATING CROP WATER STRESS INDEX (CWSI) FOR ALFALFA AND TALL FESCUE GRASS

The lack of transferability of the Crop Water Stress Index (CWSI) baselines, together with the restriction of having to make required measurements close to noon and under clear-sky conditions, are major drawbacks that restrict the use of the empirical CWSI method for irrigation scheduling. The objectives of this study were to: (1) evaluate the effect of solar radiation (Rs) on the non-water-stressed baselines (NWSBs) of alfalfa (Medicago sativa L.) and tall fescue grass (Festuca arundinacea), and (2) develop empirical equations to estimate their NWSBs, which could be applied at any time during the daytime cycle and under conditions of full canopy cover. A Bowen ratio system was used to measure 20 min averages of radiometric surface temperature, air temperature (Ta), wind speed (u2), dew point, and Rs over the two crop canopies during the 1991 growing season at Kimberly, Idaho. Using this dataset, empirical NWSBs for different Rs ranges were derived, which tended to diverge from each other as vapor pressure deficit (VPD) increased, indicating that Rs considerably affected the baselines and that its effect was more pronounced as the air got drier. Multiple regression analysis was also used to develop equations to estimate the NWSBs for the entire daytime cycle and specifically for the near-noon period. For alfalfa, the equation derived for the entire daytime cycle estimated the NWSBs as a function of Rs, VPD, Ta, u2, and plant canopy height (h) (r2 = 0.89). For grass, the equation only included Rs, VPD, Ta, and u2 (r2 = 0.89). For alfalfa, the near-noon equation included Rs, VPD, Ta, u2, and h (r2 = 0.92). For grass, on the other hand, Ta and h were not statistically significant, and the near-noon equation only included VPD, Rs, and u2 (r2 = 0.94). Since all variables that significantly affected the NWSBs for these crops were included in the equations, we expect them to be transferable to other locations; however, additional testing at other locations is needed to confirm this hypothesis.

Sprinkler Irrigation Spray Temperatures

THE temperature of irrigation water may vary from zero, where the source is a melting snowbank, to near 90 C where the source is a hot spring or well such as those along the Snake River. Water heated by nuclear reactors may provide hot water for irrigation in the future. Water of these extreme temperatures is frequently used in sprinkler irrigation. It is important to crop production and management to know the temperature of the water as it comes in contact with the crop or soil because of the effect of temperature on germination of seed; development of fruit, vegetables, and other crops; frost protection, and crop-cooling operations.

Revised FAO Procedures for Calculating Evapotranspiration: Irrigation and Drainage Paper No. 56 with Testing in Idaho

In 1998, the Food and Agriculture Organization of the United Nations (FAO) published FAO Irrigation and Drainage Paper No. 56, a revision of the earlier and widely used Paper No. 24 for calculating evapotranspiration (ET) and crop water requirements. The revision uses a single method, the FAO Penman-Monteith equation, for calculating reference evapotranspiration (ET o ). In addition to the “mean” crop coefficient (K c ) values of FAO-24, FAO-56 provides tables of “basal” crop coefficients that represent ET under conditions having a dry soil surface. Associated equations for predicting evaporation from bare soil associated with crop transpiration are based on a water balance of the soil surface layer. Comparisons of daily ET from three agricultural crops are made between lysimeter measured ET and the basal K c method of FAO-56 and the time-based basal K c procedure of Wright (1982). Standard errors of estimate and accuracies were similar between the two methods and averaged about 0.77 mm/day or 15%.

Near‐noon albedo values of alfalfa and tall fescue grass derived from multispectral data

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.

Estimation of Evaporation and Evapotranspiration during Nongrowing Seasons Using a Dual Crop Coefficient

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.