The approaches for estimating crop water requirements were based upon transpiration ratio and depth-interval-yield data. Transpiration ratio is the quantity of water required to produce a unit amount of dry matter.
Some considered dry matter to be the weight of the shoot while others considered it as the weight of the economic produce of the crop. This ratio was considered to be the water requirement of the crop but later this idea was abandoned as under field conditions dry matter production and water use are affected by different sets of factors.
In the depth-interval-yield approach different depths of irrigation were applied at different intervals. The irrigation treatment which gave the maximum yield with the minimum depth of water was taken as indicating the optimum water requirement for that particular crop.
No rational approach was followed in fixing the depths and intervals for irrigation. This resulted either under irrigation or over irrigation or sometimes delayed or premature irrigations. This approach therefore did not provide reliable criteria for determining crop water requirements.
The Energy Balance at Land Surface:
If-was gradually realised that the water use by the plants is governed primarily by the climatological parameters including the energy available at the land surface. The evapotranspiration process consists in conversion of water to vapour from liquid phase. The source of energy for this process is the radiation received from the sun.
Solar radiation reaches the outer surface of the earth’s atmosphere at a rate of 2 calories/cm2/min (langleys/min) measured perpendicularly to the beam. All the radiation received from the sun is not available at the land surface.
The sun’s radiation is partly absorbed and reflected by the atmospheric components like clouds, water vapour etc. Different types of land surfaces and vegetation reflect different amounts of energy.
Based on the principle of the conservation of energy and considering a soil column extending from the surface to some depth where vertical heat exchange is negligible, we can write –
The energy balance equation given above applies over any time period. The units of the terms in the above equation are usually expressed in calories/cm2 per unit time. Later the unit mm of water evaporated per unit time will be used for calculating evapotranspiration values. It can be shown that 1 cal/cm2/min is approximately 1 mm water evaporated per hour.
The amount of radiation energy available on a horizontal land surface is known as net radiation (Rn). It is the difference of total incoming and outgoing radiation and is given by –
Evaporation Pans:
The evaporation from a free water surface of an open pan is widely used as an indicator of the evapotranspiration of crops. Because of their convenience in operation, pans are being widely used all over the world.
Pan evaporation depends upon the dimensions, exposure, material of construction as well as on the meteorological conditions. The class A pan of the U.S. Whether Bureau is widely used as the standard pan.
The pans installed above ground receive additional energy due to absorption of radiation through the pan wall and transfer of heat between the air and the pan wall. This results in higher evaporation rates than are calculated from meteorological data.
Sunken pans might then be expected to give more reliable results, but heat exchange between pan wall and surrounding soil limit the accuracy of results obtained with them.
Empirical correlations are usually developed between the measured evaporation rate from the pan and the potential or actual evapotranspiration rates.
The difference between the pan evaporation and crop evapotranspiration are due to:
(1) The non-similarity of reflectance from crop and water surface,
(2) The wind profile over crop differs from that over a pan,
(3) Heat storage in the pan may be considerable and will contribute to more evaporation,
(4) Evaporation from pans continues at night but most plants have closed stomatal during darkness, and
(5) The pan offers no stomatal resistance to moisture loss as in plants and as such pan evaporation should only be correlated with potential evapotranspiration.
Table 12.7 summarizes selected values of K for a range of crops at various stages of growth, grown in ‘normal moisture conditions required for a maximum growth and high yields under commercial agricultural practice’.
It should be noted that any correlation between crop evapotranspiration and pan evaporation is unique to that pan, crop and location. Experimental values of crop coefficients collected from one area should, therefore, be used with caution in other areas.
Modified Approaches For Estimating ET Values:
Doorenbos and Pruitt (1977) after reviewing field data collected at several places in the world, outlined the following modified methods for determining the evapotranspiration values.
The methods given by them are:
(1) Modified Blaney-Criddle,
(2) Radiation,
(3) Modified Penman, and
(4) Pan evaporation.
All these methods calculate the reference crop evapotranspiration (ET0). The reference crop evapotranspiration is defined as the rate of evapotranspiration from an extended surface of 8 to 15 cm tall, green grass cover of uniform height, actively growing, completely shading the ground and not short of water. In order to obtain the evapotranspiration values of a particular crop, ET0 values are multiplied by the crop coefficient (Kc) for that particular crop.
1. Modified Blaney-Criddle Method:
The Blaney-Criddle factor in mm is expressed as –
Where, T is the mean of the daily maximum and minimum temperatures in °C over the month considered; p is the mean daily percentage of annual day time hours for the given month and latitude, ET0 is given in mm per day and represents the mean values over the month.
ET0 values are now determined using the relationships between f and ET0 (Fig- 12.2). These figures are developed using values of day time minimum humidity (RHmin), ratio of actual to maximum possible sunshine hours (n/N) and day time wind conditions (u) at 2 m height.
Since f is expressed in mm per day, ET0 and ET (crop) are also expressed in mm per day and represent the mean daily values for the period considered (usually one month).
2. Radiation Method:
This method is useful in areas where measurements of air temperature and sunshine or cloudiness or radiation are available. In respect of wind and humidity, only general levels are needed.
Where, Rs is the solar radiation at the ground level expressed in equivalent in mm/day, W is a weighting factor which depends upon temperature and altitude; c is a coefficients as given in Fig. 12.3.
ET0 is given in mm per day and represents the mean value for the period concerned. Solar radiation can be measured directly using instruments known as solarimeter. Where such measurements are not available, it can be estimated from bright sunshine duration records using the formula –
Where n/N is the ratio between actual to maximum possible bright sunshine hours and Ra is the extraterrestrial radiation. Values of Ra and N can be obtained from standard tables prepared for different latitudes and times of the year.
The weighting factor W includes the effect of temperature and elevation. Temperature reflects the mean temperature in °C for the period concerned. Table 12.9 gives the values of W as related to temperature and altitude.
This method is considered to be more reliable than the Blaney-Criddle approach, particularly in equatorial regions, on small islands and at high elevations.
3. Modified Penman Method:
The modified Penman method is likely to provide the most satisfactory results to predict the effect of climate on crop water requirements. This method also requires more climatic parameters than the other methods.
(a) Vapour Pressure (ea – ed):
The term ea – ed represents the influence of relative humidity. ea – ed can be calculated whether the relative humidity of the air expressed as RHmax and RHmin in percentages or as temperature readings of dry bulb and wet bulb thermometers.
The following examples indicate how ea – ed is calculated from these readings:
Case 1 – When relative humidity values are given –
Case 2 – When dry bulb and wet bulb temperatures are given –
Vapour pressure is to be expressed in m bar. If ed is given in mm Hg, it is to be multiplied by 1.33 (1013/766) to convert into m bar.
(b) Wind Function f(u):
The wind function f(u) is given by the expression-
Where, u2 is the total wind run in km/day at 2 m height. Where wind data are not collected at 2 in height, appropriate corrections for wind measurements taken at different heights may be made. The correction factors given by Doorenbos and Pruitt (1977) are given in Table 12.10.
(c) Weighting Factor (I – W):
This weighting factor takes into consideration the effect of wind and humidity on ET0. Values of (I – W) as related to temperatures and elevation can be calculated from W values given in Table 12.9.
(d) Net Radiation, Rn:
Net radiation can be directly measured using net radiometers. However, these measurements are not taken at all places. To calculate the net radiation, solar radiation Rs is calculated as described for the radiation method. The two components of the solar radiation viz., the short wave and long wave radiation are separately calculated. To obtain the net short wave radiation (Rns), Rs is corrected for the reflectivity of the crop surface a. An average value of a is taken as 0.25 and Rns is given by –
The net long wave radiation Rnl is determined using available temperature, vapour pressure and n/N ratio values. The coefficient used to describe the effect of temperature, vapour pressure and n/N values are given in Tables 12.13, 12.14 and 12.15. Using these values Rnl is given by –
(e) Calculation of ET0:
Using Eq. 12.26, ET0* is calculated. This value needs to be adjusted for day and night time weather conditions. The wind run or mean wind speed during day time hours averages about twice that occurring during the night hours. Also, the night time relative humidity values are higher than the day time values.
Variation of measured and calculated values of ET0* will result under climatic conditions not represented. ET0* values should be corrected depending upon the day and night time wind and humidity conditions. Fig. 12.4 illustrates the range of such conditions in a graphical form that can be used for correcting the calculated ET0* values.
4. Pan Evaporation:
In this method, the evaporation of water from a pan of standard dimensions is observed. The reference crop evapotranspiration ET0 in mm/day is then calculated by –
Where, Epan is pan evaporation in mm/day and Kp is the pan coefficient. The pan coefficient depends upon the type of the pan, its surroundings and also atmospheric conditions. Doorenbos and Pruitt consider two types of pans; the US Class A pan and the sunken Colorado pan.
Crop Coefficients:
In the four methods, the reference crop evapotranspiration ET0 is determined. In order to get the evapotranspiration values for particular crop, ET0 is to be multiplied by the respective crop coefficients.
Crop coefficients are affected by the crop characteristics, time of planting or sowing stage of crop development and climatic conditions including frequency of rain or irrigation. For determining the crop coefficients, the crop development is considered in four stages.
These are:
1. Initial stage – Germination and early growth when the soil surface is not covered by the crop.
2. Crop development stage – From the end of the initial stage to attainment of effective full ground cover.
3. Mid-season stage – From attainment of effective full ground cover to time of start of maturity.
4. Late season stage – From end of mid-season stage until full maturity or harvest.
Kc values are determined using the following procedure:
(i) The sowing dates, growing season and length of crop development stages are established from local information.
(ii) For the crop initial stage, the irrigation and rainfall frequency is determined. Using Fig. 12.5, Kc value is determined.
(iii) Using values in Table 12.18, Kc values for the mid-season as well as late season are selected.
(iv) Using values in Table 12.18, Kc values for the mid-season as well as late season are selected,
(v) The selected values are plotted as shown in Fig. 12.6. From the initial stage to mid-season and from mid-season to maturity straight line relation is assumed. Thus, the Kc values for any stage of the crop period are determined.
These crop coefficients are to be used with the methods outlined by Doorenbos and Pruitt. In other methods it is possible that other values of crop coefficients are used.
Example:
Computation of reference crop evapotranspiration ET0 using modified Penman Equation. Location – Delhi 280° 4′ N and altitude 228M, Period 1-10 August 1970.
The calculation procedures are cumbersome when calculations are to be made manually. However, computer programs for these calculations are available and can also be developed.
Dooronbos and Pruitt (1977) provided a computer program for calculating ET values using the methods outlined by them. The program CROPWAT (Version 5.7) calculates the Penman-Monteith reference ET and also calculates the crop water requirement with selected crop coefficients.
Water Balance Method:
This method requires complete details of the disposition of water. The water balance equation can be written as –
where W is the change in water content in the rootzone during a certain period, P is the precipitation in that period, I is the irrigation, R is the runoff, D is the deep drainage from the rootzone, and E is the combined loss due to transpiration and evaporation.
The water balance method can be applied over a small area or large area like an entire basin. In the case of large basins the groundwater changes in the basin can also be considered in the water balance equation. The main difficulty in this approach is in collecting the information needed with a reasonable accuracy.
Use of Lysimeters:
Lysimeters are large tanks filled with soil and supported on some type of weighing mechanism. The lysimeter is to be located in an area where the same types of plants are grown in the lysimeter as well as in the surrounding area. Evapotranspiration rates are obtained from weight changes of the lysimeter using the following water balance equation–
Lysimeters are generally classified as weighing type or drainage type. In the drainage type lysimeters, the inflows and drainage are measured, but changes in storage within the soil are not measured.
Weighing type lysimeters allow an evaluation of each element in the water balance equation. The changes in storage are measured by weighing the soil block. Weighing is accomplished either manometrically or by a suitable scale system.
Several factors need to be considered in constructing a lysimeter. The heat advection through the walls is to be kept to a minimum, preferably by using a circular tank and allowing as little space as possible between the lysimeter walls and the surrounding soil. To maintain a moisture gradient representative of the surroundings, adequate drainage facilities must be provided at the base of the lysimeter.
The method by which the lysimeter is filled with soil can be very important. In some soils careful backfilling by layerwise may be satisfactory while in soil particularly with variabilities, monolith blocks may be cut and fitted in the lysimeters. The lysimeters provide accurate information but because of the great care needed in their constructions they cannot be installed in large numbers and hence their use is limited.
Agro-Meteorological Observations:
Climatological parameters required for estimating evapotranspiration values are observed using different instruments. Fig. 12.8 shows a typical layout of different instruments in an Agro-meteorological observatory. Additional instruments to those shown in the figure can always be added.
A brief description of different agro-meteorological instruments is given below:
1. Sunshine Recorder:
The commonly used sunshine recorder known as the Campbell-Stokes type consists of a spherical glass ball of 7.5 cm diameter mounted on a stand with a paper chart underneath. The ball focuses the sunlight onto the paper chart resulting in burning a trace on the paper chart as the sun moves across the sky.
The length of the burn indicates the duration of sunlight. Some qualitative measure of intensity is given by the width of the burn. The position of the paper chart is to be changed to suit the season of the year. The instrument is not sensitive to diffuse radiation.
2. Wind Vane:
The wind vane indicates the direction of the prevailing winds at a particular time.
3. Anemometer:
This is used for measuring the wind speed. The cup type anemometer is commonly used. This consists of three hemispherical cups rotating due to wind. The numbers of rotations are recorded by a counter. Knowing the number of rotations in a given period, the average wind speed during that period is calculated.
4. Minimum and Maximum Thermometers:
These record the minimum and maximum temperatures occurring in a day.
5. Dry and Wet Bulb Thermometers:
The readings obtained by the pair of dry and wet bulb thermometers are useful in determining the relative humidity of the atmosphere. In the wet bulb thermometer, the bulb part is wrapped with wet muslin or a thin piece of cloth.
At usual atmospheric conditions, when evaporation takes place, the wet bulb will show a lower temperature than the dry bulb. When air is saturated the two will give the same temperature reading. The relative humidity is determined using these two readings.
All these thermometers are housed in a wooden box with openings obtained by a series of inclined battens. These boxes are known as Stevenson’s screens.
6. Rain Gauges:
Recording and non-recording rain gauges are used for measuring rainfall.
7. Soil Thermometers:
These are installed at 5 cm, 15 cm and 30 cm depth to observe the soil temperatures.
8. Pan Evaporimeter:
The commonly used open pan for observing evaporation of water is known as the US Class A type open pan. This pan is circular 121 cm (45.5 inches) in diameter and 25.5 cm (10 inches) deep. It is made of galvanized iron (22 gauge) or monel metal (0.8 mm).
The pan is mounted on a wooden open frame platform with its bottom 15 cm above ground level. The soil is built upto within 5 cm of the bottom of the pan. The pan is filled with water, 5 cm below the rim. Water is regularly changed to eliminate turbidity.
The pan is covered with a wire mesh in order to prevent the birds from using the water in the pan. A stilling well and depth gauge help in accurately observing the depth of water in the pan.
At present, combined units consisting of several equipment are available. Observations could be transmitted using electronic transmissions and can be recorded on a continuous basis.
9. Schedule of Observations:
The Indian Meteorological Department recommends the following schedule of observations in the Agro-meteorological Observatories:
Morning (7:30 a.m.) – Max. and Min. temperatures; soil temperature; dry and wet bulb temperatures; wind direction and wind speed.
Morning (8:30 a.m.) – Open pan evaporation; rainfall.
Evening (2:30 p.m.) – Max. temperature; soil temperature; dry and wet bulb temperatures; wind direction, wind speed; open pan evaporation; rainfall.
Doorenbos (1976) recommends that the time of observations should follow national practice. Where no such standards are followed, the first observation of the day should be taken at the first full hour after the latest sunrise.
For example – if in winter the latest sunrise is at 7:30 hours, the first reading should be taken at 08:30 hours throughout the year. The second observation should be two hours after local noon. Exact time of observation should be noted if there is a deviation.
Soil and Plant Factors Affecting Evapotranspiration:
The potential evapotranspiration rate (E) decreases when soil moisture suction increases i.e., as the soil moisture content tends towards the wilting point, there is a reduction in the rate of evapotranspiration.
This is due to a reduction in the potential gradient from the soil water to the leaves and hence the moisture flow rate decreases. The relative evapotranspiration (Ea/E) is also dependent upon the soil moisture retaining capacity of the soil and the extent of root system. (Ea is actual evapotranspiration).
Various moisture tension-evapotranspiration curves have been proposed. These range from a horizontal line at Ea/E = 1.0 indicating equal availability of water from field capacity to wilting point (line A) to a line joining Ea/E = 1.0 at field capacity to the point Ea/E = 0 at wilting point (line C in Fig. 12.12).
For all practical purposes, it is reasonable to assume that Ea/E is unity when the soil moisture storage is at maximum and that the ratio equals zero when the available soil moisture storage is zero. The portion of the Ea/E curve is between these two limits depends upon the crop and soil moisture characteristics and needs to be determined for each situation.
Plant population and the stage of growth also influence evapotranspiration. Evapotranspiration increases gradually from planting time to maturity and thereafter it decreases gradually. The gradual increase is due to an increase in plant cover.
Evaporation from most of the bare soils decreases rapidly within a day or two after irrigation whereas transpiration from plants is maintained at the same level for several days. Because of this reason, evapotranspiration increases as percent cover increases. The gradual decrease with the stage of growth at the end of growing season is essentially due to plant physiological factors.