Crop Evapotranspiration

Abstract

Crop evapotranspiration is a function of both weather (reference ET) and the growth stage of the plant. The ratio between reference ET and crop ET is the crop coefficient, which changes during the season with crop physiological changes. At the time of planting, most ET is evaporation from the bare soil surface, but as the canopy matures ET is predominantly transpiration from the crop (Fig. 6.1). The single crop coefficient combines evaporation and transpiration. A theoretically more accurate approach divides the evaporation and transpiration into two separate coefficients (dual crop coefficient). Both methods are extensively described in the Food and Agriculture Organization Irrigation and Drainage Paper No. 56 (FAO 56). Crop coefficients are normally calculated based on days after planting, but they can also be calculated based on heat units after planting. Heat units are the integrated temperature and time after planting. This chapter also includes a Fourier series approach to modeling crop coefficients and weather. Finally, the chapter includes a brief introduction to the use of remote sensing for irrigation scheduling.

Questions

• 1. Reference ET₀ is 10 mm/day for 1 week, and the crop coefficient is 0.5. What is the depth of water required during 1 week?

• 2. Reference ET₀ is 7 mm/day, pan evaporation is 8 mm/day, and measured crop evapotranspiration is 5 mm, as measured by a lysimeter. What are the pan and reference ET₀ crop coefficients?

• 3. Explain the difference between the single crop coefficient and the dual crop coefficient.

• 4. Explain the spikes in transpiration rate in Fig. 6.4 and the average crop coefficient in Fig. 6.5.

• 5. Find the Kcb values for winter wheat (dual crop coefficient) in FAO56 Table 17, and adjust K_cb-mid for average minimum relative humidity during mid-season and late season growth stages equal to 20 % and 30 %, respectively. Average wind speed at 2 m elevation is 2 m/sec during mid and late season growth stages. There is less than 10 % ground cover during the initial phase. There is high grain moisture at harvest. The crop is grown in California. Plot the linearized crop coefficient curve for the dual component model.

• 6. Repeat question 5, but plot the single crop coefficient curve (FAO Table 12) for winter wheat. Average depth of irrigation during the initial phase is 50 mm. Average number of days between irrigations during the initial period is 20 days. Minimum relative humidity is 20 % and average wind speed is 2 m/sec. The crop is machine harvested (late season K_c). During the early season, the crop is irrigated every 10 days, irrigation depths are greater than 40 mm, and the ET₀ is 7 mm/day. Also plug values into the Example 6.10 worksheet in order to solve the problem.

• 7. Calculate the single day crop basal transpiration for winter wheat 70 days after planting. Reference ET₀ is 7 mm/day and the crop stress coefficient is 0.8. Relative humidity is 20 %, and wind speed is 3 m/sec at 3 m elevation.

• 8. Plot the winter wheat root growth curve as defined in FAO56. Initial depth is 0.4 m and the final depth is 0.8 m.

• 9. What is the reason that soil evaporation + basal transpiration cannot exceed K_c-max?

• 10. Calculate K_{c − max} if relative humidity is 20 %, crop height is 0.5 m, and wind speed is 2 m/sec.

• 11. Explain the difference between REW and TEW.

• 12. Field capacity is 20 % and permanent wilting point is 10 %. The depth of the surface layer is 0.11 m. REW = 7 mm. Calculate TEW. Calculate K_r for the surface layer depletion equal to 4 mm.

• 13. Alfalfa has a low ET just after cutting and high ET just before cutting. For arid conditions with moderate wind, calculate alfalfa evapotranspiration just before and just after cutting if reference ET₀ is 10 mm/day. Use FAO 56 Table 12.

• 14. During the first week after planting watermelons, t_max and t_min are 32 °C and 8 °C, respectively. Calculate the number of growing degree days accumulated after 1 week.

• 15. Calculate heat unit Fourier series K_c and ET_c curves for watermelon and corn for (Instructor selects year and location) weather data. You can download this data with the Chapter 5 ET Calculator – Active Year Weather worksheet from a city in Arizona or use weather from your home state. Click the Run Weather Form button. Calculate the Fourier W coefficients for the Tmax, Tmin and ETo curves in the Chapter 5 ET Calculator – Fourier T and ETo worksheet. Then, insert your W coefficients for Tmax, Tmin and ETo into the Chapter 6 Crop ET and scheduling – Fourier T and ETo worksheet. Then, copy your Tmax, Tmin and ETo values into the Chapter 6 Crop ET and scheduling – ETo and temp – ch 5 worksheet. Soil is heavy-textured.

• 16. As in Example 6.6, calculate corn K_c and ET_c 50 days after planting by hand (with a calculator) and show that calculated K_c and ET_c in the spreadsheet agree with the equations.

• 17. According to the Fourier series GDD equations, calculate the rooting depth for melons when GDD = 500 and 1,000. AWC is 10 %, MAD is 0.5, and ET_c is 7 mm/day at the first rooting depth and 9 mm/day at the second rooting depth. What is the required frequency of irrigation at the two rooting depths.

• 18. For the 2008 Fourier series Tucson weather data, and the Fourier series crop coefficient for alfalfa, input the cutting cycle with peak ET into the High frequency alfalfa Irr. Worksheet. Then determine the required depth of irrigation per day with a center pivot irrigation system in order to keep up with crop water needs. Check to make sure that the percent depletion does not exceed 50 %. Use TAW = 140 mm. Assume a leaching fraction of 10 % and irrigation efficiency of 90 % in order to calculate the required gross application rate (divide net application rate in Worksheet by efficiency and (1 – LF)).

• 19. Calculate the CWSI for potatoes if the air temperature is 25 °C, canopy temperature is 24 °C, and actual vapor pressure is 1.32 kPa. Determine whether irrigation is needed.