Potential evapotranspiration

Exercise objective:

·   Selecting evapotranspiration modules and availability of weather data.

Suggested reading:

Chapters 2.3, 4.1, 7.5 of the book, ORYZA2000: modelling lowland rice.

Exercise: 

In the potential production mode, ORYZA2000 also calculates the reference (for 10cm short grass cover) evapotranspiration rate, ETD (mm water d-1), the potential evaporation rate of the soil, EVSC (mm water d-1), and the potential transpiration rate of the specific crop, TRC (mm water d-1). Details on the calculation procedures are given in Chapter 4.1 of the book “ORYZA2000: modelling lowland rice”. Potential evapotranspiration is assumed to take place under conditions of unlimited supply of water, hence no water balance is needed (compare with “Chapter IV: Water-limited production”). Three options to calculate potential evapotranspiration are available: Penman, Priestley-Taylor and Makkink. The choice for either of the three is governed by: (i) user preference, and (ii) availability of weather data. The Penman method requires the availability (in the weather data file) of five weather variables: radiation, maximum and minimum temperature, wind speed and vapour pressure. The Priestley-Taylor and Makkink methods only require radiation and minimum and maximum temperature. In ORYZA2000, the reference ETD of the grass cover is used to calculate potential evaporation of the soil (EVSC) and potential transpiration of the rice crop (TRC).

Ex-II.15. Change in the CONTROL.DAT file the experimental data file to EXPLORE.DAT and verify that the selected method to calculate evapotranspiration is Penman: ETMOD = 'PENMAN'. Run the model and make a graph of ETD, EVSC and TRC versus time (Figure II.8). Explain the patterns of EVSC and TRC. A. EVSC is high early in the season when the crop is small and the soil/water layer receives direct sunlight for evaporation. As the crop grows, it increasingly shields the soil/water layer from sunlight and EVSC decreases. The pattern for TRC is the reverse: it increases from low levels early in the season when the crop is small and has few leaves to transpire, to high levels later in the season when the canopy is closed and fully transpiring. TRC exceeds ETD because of the high leaf area index values that rice reaches.

Figure II.8. Simulated reference evapotranspiration, ETD, potential evaporation of the soil, EVSC, and potential transpiration of the crop, TRC, as a function of time (all in mm water d-1).

Ex-II.16.  Make reruns with Priestley-Taylor and Makkink as the selected evapotranspiration options, respectively:

ETMOD = 'PRIESTLEY TAYLOR'   ! Priestley-Taylor

ETMOD = 'MAKKINK'            ! Makkink

Make a graph of TRC versus time for all three runs (Figure II.9) and notice the small differences in TRC values. Generally, the Penman method is considered the most accurate, and is therefore preferred if all five weather variables are available.

Ex-II.17. In EXPLORE.DAT, change the weather data file used from PHIL1.992 to PANT1.992 (CNTR   = 'PANT'). PANT1.992 contains weather data from the location Pantnagar in India for 1992; study the contents of this file. Run ORYZA2000 and examine the result; read the WEATHER.LOG file.

Q: Why is the simulation halted already on day 4? 

A. In EXPLORE.DAT, ETMOD = 'PENMAN', but the weather data file PANT1.992 has missing values for wind speed and vapour pressure, hence the calculations cannot be performed. Now change the selected method to calculate evapotranspiration to either Priestley-Taylor or Makkink, run ORYZA2000 and examine the results. There are still errors in the WEATHER.LOG report but these have no effect, since ORYZA2000 now does not require wind speed or vapour pressure to calculate evapotranspiration.

Ex-II.18. Restore in EXLORE.DAT the selection of ETMOD to Penman and the weather data file to PHIL1.992.

Figure II.9. Simulated potential transpiration of the crop, TRC (mm water d-1) with three different calculation methods.