Watershed Hydrology: Lesson 13 Evapotranspiration

Lesson 13 Evapotranspiration

13.1 Transpiration

Transpiration consists of the vaporization of liquid water contained in plant tissues and the vapour removal to the atmosphere. Crops predominately lose their water through stomata. These are small openings on the plant leaf through which gases and water vapour pass. The water, together with some nutrients, is taken up by the roots and transported through the plant. The vaporization occurs within the leaf, namely in the intercellular spaces, and the vapour exchange with the atmosphere is controlled by the stomatal aperture. Nearly all water taken up is lost by transpiration and only a tiny fraction is used within the plant.

Transpiration, like direct evaporation, depends on the energy supply, vapour pressure gradient and wind. Hence, radiation, air temperature, air humidity and wind terms should be considered when assessing transpiration. The soil water content and the ability of the soil to conduct water to the roots also determine the transpiration rate, as do water logging and soil water salinity. The transpiration rate is also influenced by crop characteristics, environmental aspects and cultivation practices. Different kinds of plants may have different transpiration rates. Not only the type of crop, but also the crop development, environment and management should be considered when assessing transpiration.

13.2 Evapotranspiration (ET)

Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes. Apart from the water availability in the topsoil, the evaporation from a cropped soil is mainly determined by the fraction of the solar radiation reaching the soil surface. This fraction decreases over the growing period as the crop develops and the crop canopy shades more and more of the ground area. When the crop is small, water is predominately lost by soil evaporation, but once the crop is well developed and completely covers the soil, transpiration becomes the main process. In Figure 1 the partitioning of evapotranspiration into evaporation and transpiration is plotted in correspondence to leaf area per unit surface of soil below it. At sowing nearly 100% of ET comes from evaporation, while at full crop cover more than 90% of ET comes from transpiration.

Fig.13.1. The partitioning of evapotranspiration into evaporation and transpiration over the growing period for an annual field crop (Richard,1998 )

The evapotranspiration rate is normally expressed in millimetres (mm) per unit time. The rate expresses the amount of water lost from a cropped surface in units of water depth. The time unit can be an hour, day, decade, month or even an entire growing period or year.

13.3 Measurement of Evapotranspiration

The principal methods for direct measurement of evapotranspiration are:

1) Lysimeter experiment

2) Field experimental plots

3) Soil moisture depletion studies

4) Water balance method

13.3.1 Lysimeter

A lysimeter is a special watertight tank containing a block of soil and set in a field of growing plants. The plants grown in the lysimeter are the same as in the surrounding field. Evapotranspiration is estimated in terms of the amount of water required to maintain constant moisture conditions within the tank measured either volumetrically or gravimetrically through an arrangement made in the lysimeter. Lysimeters should be designed to accurately reproduce the soil conditions, moisture content, type and size of the vegetation of the surrounding area. They should be so hurried that the soil is at the same level inside and outside the container. Lysimeter studies are time-consuming and expensive.

13.3.2 Field Experimental Plots

Measurement of water supplies to the field and changes in soil moisture content of the field plots are sometime more dependable for computing seasonal water requirement of crops than measurement with lysimeters which do not simulate field conditions. The seasonal water requirements are computed by adding measured quantities of irrigation water, the effective rainfall received during the season and the contribution of moisture from the soil. Field water balance may be expressed by the following relationship:

(13.1)

Where,WR is seasonal water requirement (mm), IR is total water applied (mm), ER is seasonal effective rainfall (mm), M_bi and M_ei are the moisture percentage at the beginning and end of the season in the i_thlayer of soil, A_i is the apparent specific gravity of the i_thlayer of soil, D_i is the depth of the i_thlayer of soil within the root zone (mm) and n is the number of soil layer in the root zone D.

13.3.3 Soil Moisture Depletion Studies

The soil moisture depletion method is usually employed to determine the consumptive use of irrigated field crops grown on fairly uniform soils when the depth to the ground water is such that it will not influence the soil moisture fluctuation within the root zone.

Where, u is the water use from the root zone for successive sampling periods or within one irrigation cycle (mm), n is the number of soil layers sampled in the root zone depth D, M_1i and M_2i are the soil moisture percentage at the time of the first and second sampling in the i_th layer respectively, A_iis the apparent specific gravity of the i_thlayer of soil and D_i is the depth of the i_thlayer of soil (mm).

Seasonal consumptive use (Cu = Σu) is calculated by assuming consumptive use values of each sampling interval. A correction is made by adding PET values for accelerated water loss for the intervals(s) just after irrigation and before soil moisture sampling.

13.3.4 Water Balance Method

Water balance method is also called the inflow-outflow method, is suitable for large areas (watersheds) over long period. It may be represented by the following hydrological equation;

Precipitation = Evapotranspiration + Surface Runoff + Sub-surface drainage + change in soil water content

This method necessitates adequate measurement of all factors, expect evapotranspiration. The value of evapotranspiration is computed from the measured data.

13.4 Determination of Evapotranspiration

Owing to the difficulty in obtaining accurate direct measurement of pan evaporation under field conditions, evaporation is often predicted on the basis of climatological data. The approaches followed are to relate the magnitude and variation of evapotranspiration to one or more climatic factors (temperature, day length, humidity, wind, sunshine etc.). The more commonly used empirical formulae in estimating evapotranspiration are:

a) Blaney-Criddle Method

b) Thornthwaite Method

c) Hargreaves’ Method

The Blaney-Criddle method is recommended for periods of one month or longer.

13.4.1 Blaney-Criddle Method

This method requires the use of only two factors, temperature and information of day light hours which is a factor based purely on the latitude of the place. Using Blaney-Criddle approach, potential evapotranspiration can be expressed as follows, in metric unit:

(13.3)

Where, PET = potential evapotranspiration, mm of water per day (mean value over the month)

P= monthly percent of total day time hours of the year

T= mean monthly temp. In °C (Average of daily max and minvalues)

The seasonal consumptive use of a crop can be determined from the following relationship,

(13.4)

Where, U = seasonal consumptive use of water by the crop for a given period (mm/inches)

u = monthly consumptive use (mm)

K = empirical seasonal consumptive use consumptive us for the growing season

F = sum of the monthly consumptive use factors (f) for the growing season

k = empirical consumptive use crop coefficient for the month (u/f)

f = value of monthly PET in mm

13.4.2 Thornthwaite Method

Thornthwaite method is based on the assumption of an exponential relationship between mean monthly temperature and mean monthly consumptive use.

(13.5)

Where, e = unadjusted PET (cm per month)

t = mean air temperature (°C)

I = annual or seasonal heat index, the summation of 12 values of monthly heat indices (i) when, i = ( t / 5 )^1.514

13.4.3 Hargreaves’ Method

Hargreaves based on his work on data from grass lysimeter, proposed the following relationship to estimate ET,

Where, PET = reference crop potential consumptive use

t = mean daily temperature (°C)

R_s= incident solar radiation in langlay/day, it can be calculated using the following relationship,

(13.7)

Where, S is the percent possible sunshine hour and R_so is the clear daysolar radiation in langlay/day.

13.4.4 FAO Penman-Monteith Method

The FAO Penman-Monteith method is used to estimate reference evapotranspiration. The equation is:

Where, ET₀ = reference evapotranspiration [mm day^-1]

R_n = net radiation at the crop surface [MJ m^-2 day^-1]

T = mean daily air temperature at 2 m height [°C]

u₂ = wind speed at 2 m height [m/s]

e_s = saturation vapour pressure [kPa]

e_a = actual vapour pressure [kPa]

e_s - e_a = saturation vapour pressure deficit [kPa]

Δ = slope of vapour pressure curve [kPa°C^-1]

Υ = psychometric constant [kPa°C^-1]

The reference evapotranspiration, ET₀, provides a standard to which:

a) Evapotranspiration at different periods of the year or in other region can be compared.

b) Evapotranspiration of other crops can be related.

References

Richard G. Allen, Luis. S. Pereira, Dirk Raes and Martin Smith. (1998) FAO Irrigation and Drainage PaperNo-56. Crop Evapotranspiration (Guidelines for computing crop water requirements)

Michael, A.M. (2010). Irrigation: Theory and Practice. VikasPublishing House Pvt. Ltd., Chapter 10.