Measurement of evapotranspiration

This presentation will provide knowledge on measurement of evapotranspiration by different methods.

1. Measurement of Evapotranspiration Vijitha Vikneshwaran Lecturer temporary Faculty of Technology University of Jaffna

2. Introduction Evapotranspiration (ET) is the combination of two separate processes whereby water is lost from the, Soil surface by evaporation and Crop by transpiration It is one of the most important components of water cycle. The accurate measurement of ET in field is importance for, Quantifying soil hydrological processes Making appropriate decisions regarding irrigation 2

3. Evaporation and transpiration occur simultaneously. There is no easy way of distinguishing between the two processes . 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 crop canopy shades most 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. Introduction, cont ….. 3

4. 4 Introduction, cont ….. Figure 01: Partitioning of ET

5. Factors affecting the ET Crop factors Crop type Variety of the crop Development stage Differences in resistance to transpiration Crop height Crop roughness Reflection Ground cover Crop rooting characteristics 5

6. Weather parameters Radiation Air temperature Humidity Wind speed Management and environmental conditions Soil salinity Land fertility Application of fertilizers Presence of hard or impenetrable soil horizons Control of diseases and pests Poor soil management 6 Factors affecting the ET, cont ….

7. Other factors Ground cover Plant density Soil water content 7 Factors affecting the ET, cont ….

8. Reference crop evapotranspiration ( ETo ) It is the evapotranspiration rate from a reference surface, not short of water. The reference surface is a hypothetical grass reference crop with specific characteristics such as, Height: 0.12 m Fixed surface resistance : 70 s m -1 Albedo: 0.23 8

9. As water is abundantly available at the reference evapotranspiring surface, soil factors do not affect ET. The only factors affecting ETo are climatic parameters. Consequently, ETo is a climatic parameter and can be computed from weather data. ETo can be measured by Penman- Monteith method. ETo can also be estimated from pan evaporation however special precautions and management must be applied . Any how use of pans to predict ETo for periods of 10 days or longer may be warranted. (Refer the ppt : Measurement of evaporation) 9 Reference crop evapotranspiration ( ETo ), cont …..

10. Crop evapotranspiration under standard ( ETc ) and non-standard conditions( ETc adj ) ETc is the evapotranspiration from disease-free, well-fertilized crops that grown in large fields under optimum soil water conditions and achieving full production under the given climatic conditions. ETc adj is the evapotranspiration from crops grown under management and environmental conditions that differ from the standard conditions. 10

11. It can be measured by direct or indirect methods. Climate and crop data are used to estimate the evapotranspiration in indirect measurement by theoretical and empirical equations. Direct measurement involves isolating a portion of the crop from its surrounding and determining ET by measurement. It can be measured by four methods: Lysimeter method Field Experimental plot Water balance method Soil Moisture Depletion Study Measurement of Evapotranspiration 11

12. Lysimeter It is a device used to measure the amount of actual ET which is released by plants. There are two types, Weighing type N on weighing type 12

13. Working principle of lysimeter A lysimeter is a device introduced in the ground, filled with the same soil of the study area and with vegetation. It is used to measure the reference ET or the crop ET. The ET measurement is determined by the water balance of the devices. In weighing type lysimeter , there is usually a weighting scale at the bottom of the lysimeter . 13

14. Based on the scale, the amount of water evapotranspirated in the system can be determined in terms of change in mass. In non weighing type of lysimeter , a water drain system is used to estimate the drainage water.  So that ET can be determined by deducting the drainage water from the total water input. 14 Working principle of lysimeter , cont …..

15. Methodology Needed instruments are, Plastic container Soil Plant Tensiometer Measuring cylinder Water Scale The designed lysimeter should have, Storage tank Strainer unit D rainage unit C ollection unit   15

16. Experimental Set up- Lysimeter 16 Figure 02:Lysimeter

17. Methodology, cont … Procedure Select suitable location to install the lysimeter . Select 100 liters of plastic container to use as lysimeter Calculate the cross sectional area of the container Make drainage hole and connect to the collecting unit Prepare the bottom of the lysimeter with the free drainage system Setup strainer unit to prevent the movement of soil Fill the container with the soil 17

18. Allow the soil for settlement around two weeks Select good healthy plant and transplant into the lysimeter Add known volume water in to the lysimeter and measure the volume of drained water Maintain the tensiometer reading as zero by the application of known quantity of water Allow the set up for 24 hours in the field Measure the volume of drainage Apply the water balance equation and estimate the evapotranspiration. 18 Methodology, cont …

19. Water balance equation In weighing type lysimeter the water balance equation is, Input = Output P + I = ET +RO –D  S Where, ET = Evapotranspiration P = Precipitation I = Irrigation water D = Excess water drained from bottom S = weight change (Increase or decrease in storage of soil moisture) RO = Zero In non weighing type lysimeter the water balance equation is, Input = Output P + I = ET +RO –D Note: Tensiometer reading should be zero always to ensure the field is in field capacity. 19

20. Demerits Lysimeters are expensive. Operation and maintenance require special care The representativeness of the ET measured by lysimeters is apt to be suspect for, Different crop densities Crop heights R oot characteristics S oil water and nutrient statuses Soil profile structures between inside and outside lysimeters . 20

21. Variations of ET in a lysimeter and field condition Compared to large-scale lysimeters , pot experiments and micro lysimeters are widely used in studies of ET. Because these are low-cost and easy-to-use tools. But the limited amount of soil available to the roots, plant growth and ET in pots can be varies from field. Reduction in the crop yield or biomass due to water or nutrient stress can occur more frequently in pots. Even a minor difference in crop density between pots and lysimeters can result in a considerable difference of ET between them.   21

22. Soil water balance method This method consists of assessing the incoming and outgoing water flux into the crop root zone over some time period. Inputs of water to the root zone are Irrigation (I) and rainfall (P) Part of I and P might be lost by surface runoff (RO) and by deep percolation (DP) that will eventually recharge the water table. Water might also be transported upward by capillary rise (CR) from a shallow water table towards the root zone Some time the water even transferred horizontally by subsurface flow in ( SF in ) or out of ( SF out ) the root zone. 22

23. In many situations, SF in and SF out are minor and can be Ignored except under large slopes. Soil evaporation and crop transpiration deplete water from the root zone. If all fluxes other than ET can be assessed, the ET can be calculated by using below water balance equation. ET = I + P − RO − DP + CR ± Δ SF ± Δ SW Limitations: Some fluxes such as SF, DP and CR are difficult to assess and short time periods cannot be considered. 23 Soil water balance method, cont ….

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