Rl Snyder

An evaluation of common evapotranspiration equations

Abstract A comparison is made between the Pruitt and Doorenbos version of an hourly Penman-type equation, the Food and Agriculture Organization (FAO) hourly Penman-Monteith equation, and an independent measure of reference evapotranspiration (ET0) from lysimeter data. Reducing the canopy resistance improved the hourly FAO Penman-Monteith estimates. Daytime soil heat flux density is estimated as 10% of net radiation in the FAO hourly Penman-Monteith equation; however, the measured soil heat flux density under grass that was never shorter than 0.10 m in this study was between 3% and 5% of net radiation. The daytime totals of hourly ET0 from the hourly Penman-Monteith and Pruitt-Doorenbos equations and ET0 from the 24-h FAO Penman-Monteith equation were computed using data from five Italian and five Californian stations. A comparison showed that all of the equations gave acceptable results. The Pruitt-Doorenbos equation may slightly over-estimate ET0 in conditions of summertime cold air advection.

Hand calculating degree days

Abstract Degree day accumulations can be used by growers to monitor the development of biological processes and thus can be used in crop and pest management. The adoption and use of degree days by individual growers and agricultural advisors, however, has been limited partly because of computational difficulties involved in the derivation of degree days. A method is described that allows for the calculation of degree days with any threshold temperature(s) using simple arithmetic operations and a table of normalized areas from the trigonometric sine curve method of calculating degree days. The technique described will assist agriculturalists with very limited mathematical skills and/or computational facilities to adopt the use of degree days in their on-farm management.

Estimating sensible and latent heat flux densities from grapevine canopies using surface renewal

Fine-wire thermocouples were used to measure high-frequency temperature above and within canopies and structure functions were employed to determine temperature ramp characteristics, which were used in a fundamental conservation of energy equation to estimate sensible heat flux density. Earlier experiments over dense, tall, and short canopies demonstrated that the surface renewal method works, but requires a correction for uneven heating (e.g. D0.5 for tall, and D1.0 for short canopies). For sparse canopies, the calibration factor was unknown. Experiments were conducted in grape vineyards in California and Italy to determine whether the surface renewal method works in a sparse canopy and to determine if calibration is necessary. Surface renewal data were collected at several heights in the canopies and these were compared with simultaneous 1-D sonic anemometer measurements. The results indicated that the surface renewal technique provides good estimates of sensible heat flux density under all stability conditions without the need for calibration when the data are measured at about 90% of the canopy height. The values were generally within ca. 45 W m 2 of what was measured with a sonic anemometer. Separating the canopy into two layers provided even more accurate estimates of sensible heat flux density without the need for calibration. The best results were obtained when the lower layer was below the bottom of the vegetation and the upper layer included the vegetation. When combined with energy balance measurements of net radiation and soil heat flux density, using a thermocouple and the surface renewal technique offers an inexpensive alternative for estimating evapotranspiration with good accuracy.

Surface renewal analysis for sensible heat flux density using structure functions

Surface renewal (SR) analysis was used to estimate the sensible heat flux density (H) over different crop canopies (grass, wheat, and sorghum) and the results were compared with eddy covariance measurements. High-frequency temperature traces showed ramp-like structures, and structure functions were used to determine the mean amplitude and duration of these ramps. The ramp characteristics were used to estimate H. A wide range of sensible heat flux density conditions were observed. The accuracy was acceptable, but was dependent on the measurement height, the wind shear at the measurement level, and the time lags used in the structure functions. The use of surface renewal H values in energy balance determination of λE can give results nearly as accurate as those obtained using a sonic anemometer.

Correcting soil water balance calculations for dew, fog, and light rainfall

In Mediterranean climates, adoption and use of the ET-based scheduling method is limited to regions characterized by considerable contributions to evapotranspiration from fog interception, dew, and light rainfall. While the crop evapotranspiration is often accurately estimated, the water balance is frequently in error because a considerable portion of the energy expended is used to vaporize water from the plant surfaces rather from inside the leaves (i.e., transpiration). Growers in regions with considerable fog, dew, and light rainfall are hesitant to use ET-based scheduling because the cumulative crop evapotranspiration between irrigations is often considerably higher than the soil water depletion. A correction for these surface contributions is clearly needed to improve the water balance calculations and to enhance adoption of the ET-based scheduling method. In this paper, we present a simple, practical method to estimate the contribution of fog interception, dew, and light rainfall to daily crop evapotranspiration, and we show how to use the method to improve water balance calculations.

Crop Coefficients: A Literature Review

AbstractRecently presented climate change scenarios include increasing demand for water, due to both increase in temperature and variations in precipitation pattern. Since crop water requirements are related to climate and irrigation scheduling depends on crop evapotranspiration (ETc), it is crucial to have tools that allow the estimation of ETc in a reliable way. ETc is often calculated as the product of two factors, the reference evapotranspiration (ETo) and crop coefficient (Kc) values that convert ETo to ETc. This paper reviews Kc values, measured in different parts of the world and in different climates, with the aim to offer a practical tool for selecting the most appropriate Kc values for water balance irrigation scheduling, which result in more efficient irrigation. More than 100 scientific papers were studied and summarized in a report, accompanied by an extensive Kc database. The Kc report and database are available in the “Supplemental Data” section of this paper.

Correcting Midseason Crop Coefficients for Climate

AbstractIt is well known that crop coefficients are not necessarily transferrable from one location to another, for a variety of reasons. For example, the United Nations Food and Agricultural Organization (UN FAO) 24 publication on evapotranspiration listed ranges of midseason crop coefficients for particular crops, depending on wind speed and humidity. In the more recent UN FAO 56 publication, an equation was presented that adjusted crop coefficients for wind speed, humidity, and crop height. Climate correction is important for sharing and adjusting crop coefficient data. However, it was found that the FAO 56 equation gave inaccurate corrections for a crop having the same characteristics as the reference crop surface, and it is likely that it also will be inaccurate for other crop surfaces. Consequently, the aim of this research was to develop and test a new method to correct midseason crop coefficients for climate differences. Climate data were used from the California Irrigation Management Information ...