ABE 31 Introduction to MAGMT.pdf

ABE 31: Introduction to Water Management and Irrigation  Chapter 3: RAINFALL AND EVAPOTRANSPIRATION Topics Rainfall Evapotranspiration Methods of Estimating Evapotranspiration Intended Learning Outcome Discuss rainfall and evapotranspiration concepts D...

ABE 31: Introduction to Water Management and Irrigation  Chapter 3: RAINFALL AND EVAPOTRANSPIRATION Topics Rainfall Evapotranspiration Methods of Estimating Evapotranspiration Intended Learning Outcome Discuss rainfall and evapotranspiration concepts Discuss the method of evaluating evapotranspiration Estimate the crop evapotranspiration  THE HYDROLOGIC CYCLE Precipitation Condensation Leeward Side of the Mountain Windward Side of the Mountain Infiltration, Seepage Deep Percolation & Recharge Rainfall Rainfall is a term used to refer to water falling in drops after condensation of atmospheric vapor. The term also used to refer to the quantity of precipitation falling in an area within a given period of time. It is essential for the replenishment of freshwater resources, the nourishment of plant life, and the functioning of ecosystems. Adequate rainfall is vital for agriculture, while excessive or prolonged heavy rainfall can lead to flooding, landslides, and other natural disasters.  Amount of Rainfall The amount of rainfall, often referred to as precipitation, is typically measured in various units of length or depth over a specific period of time. The amount of rainfall can vary widely from one location to another and from one weather event to another. This container is placed horizontally on an Imagine an open square container, 1 meter open area in a field. wide, 1 meter long and 0.5 high.  It can be assumed that the Assume that the pan has 10mm of surrounding field has also received a water in it when the rain stops. uniform water depth of 10mm. The volume of water collected in the pan is: V (m3)= l(m) x w(m) x d(m) In terms of volume, with a rainfall of = 1m x1mx 0.010m 10mm, every square meter of the field = 0.01m3 0r 10 liters receives 0.01 m3 or 10 liters of rain water. Factors affecting the amount of rainfall: Latitude Temperature Moisture Air Masses Frontal activity Differential Heating Mountain Barriers Distribution of Land and Water  Rainfall Intensity The rainfall intensity is the depth of water (in mm) received during a shower divided by the duration of the shower (in hours). It is expressed in millimeters of water depth per hour (mm/hr). For example, a rain shower lasts 3.5 hours and supplies 35mm of water. Figure A. Low Intensity Suppose the same amount of water 35mm is supplied in 1 hour, thus by a shower of higher intensity (see figure B). Figure B. High Intensity  Rainfall Distribution refers to the pattern or spatial variation of rainfall across a specific area or region. It involves the assessment of where, when, and how much rain falls within that area. Understanding rainfall distribution is crucial for various purposes, including agriculture, water resource management, urban planning, and disaster preparedness. Fig. 1a. 100 mm rainfall, poorly distributed over one month Fig. 1b. 100 mm rainfall, evenly distributed over one month  Effective Rainfall Refers to the portion of rainfall that actually contributes to soil moisture and available for plant use or runoff into streams, rivers and groundwater. The precipitation falling during the growing period of a crop that is available to meet the evapotranspiration needs of the crop. Ineffective rainfall is that portion which is lost by surface runoff, unnecessary deep percolation losses, the moisture remaining in the soil after the harvest of the crop and is not useful for the next season crop. Pe = 0.6P-10 if P 75mm  Effective Rainfall Effective rainfall(8(=(1)-(4)-(5)-(7) Example: Estimate the effective rainfall in mm/month if the rainfall is 60 mm/month. From Table 6.Precipitation and Effective Precipitation Pe) in mm/month Table 6 it can be seen that the effective rainfall P Pe P Pe is 26 mm/month. This means that out of 60 (mm/month) (mm/month) (mm/month) (mm/month) mm/month, some 26 mm can be used by the 0 0 130 79 plants; and it is estimated that the remaining (60 10 0 140 87 - 26 =) 34 mm is lost through deep percolation 20 2 150 95 and run-off. 30 8 160 103 40 14 170 111 Question: Determine the effective rainfall for 50 20 180 119 the following monthly rainfall figures: 65, 60 26 190 127 210,175 and 5mm 70 32 200 135 Answer: (refer Table 6) 80 39 210 143 P (mm/mo) Pe (mm/mo) 90 47 220 151 65 29 100 55 230 159 210 143 110 63 240 167 175 115 120 71 250 175 5 0  Factors influencing effective rainfall 1. Climate -determines the amount, intensity and distribution of rainfall 2. Soil texture -in coarse textured soil, water infiltrates quickly 3. Soil structure -greatly influences the infiltration rate and the effective rainfall 4. Depth of the rootzone -primarily dependent on the type of crop 5. Topography -the slope and shape of the land influence runoff 6. Initial soil moisture content -if the soil is already saturated due to previous rainfall or high groundwater levels, it is less likely to absorb additional water, leading to increased runoff and reduced effective rainfall. 7. Irrigation methods -there are different methods of irrigation and each method has a specific influence on the effective rainfall. Effective rainfall and depth of the rootzone Effective rainfall and topography Effective rainfall and initial soil moisture content Fig. 2a+ b + c. Effective rainfall and Irrigation methods Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidity affects rate of evaporation of water. Fig. 3a. container with 10mm of water Fig. 3b. After 24 hours, 6mm of water is left in the container Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. Evaporation from plant surface is referred as transpiration. Evapotranspiration The evapotranspiration of a crop is the total amount of soil water used for transpiration by the plants and evaporation from the surrounding soil surface. The evapotranspiration is commonly expressed in millimeters of water used per day (mm/day) or per week (mm/week) or per month (mm/month)  Factors influencing crop evapotranspiration Factor Effect on crop evapotranspiration High Low Climate Hot Cool Dry Wet Windy No wind No clouds cloudy Crop Mid/ late season Initial or ripening Dense plant spacing Wide plant spacing Soil moisture Moist dry  Types of Evapotranspiration Evapotranspiration may be classified as: Evapotranspiration Potential Reference Actual Evapotranspiration Evapotranspiration Evapotranspiration Evapotranspiration concepts Potential Evapotranspiration (PET) Concept given by Thornthwaite and Penman is the highest rate of evapotranspiration by a short and actively growing crop with abundant foliage's completely shading the ground surface with abundant soil water supply under a given climate. refers to the maximum water loss from the crop field. often used as a reference point for estimating the actual evapotranspiration (ETa) or the amount of water that is actually lost from the Earth's surface due to evaporation from soil and water surfaces and transpiration from plants. Penman- Monteith equation is one of the most widely used formula to calculate potential evapotranspiration. Evapotranspiration concepts Reference crop Evapotranspiration (ET0) It is the evapotranspiration rate from a reference surface, not short of water. Reference surface is a hypothetical grass refence crop with specific characteristics as. height of 0.12m Fixed surface resistance of 70 sm-1 the albedo of 0.23 ET0 provides a standardized measure of potential evapotranspiration and serves as a valuable tool for crop irrigation scheduling and water resource management in agriculture. Evapotranspiration concepts Actual crop Evapotranspiration (ETa) is a measure of the actual amount of water that is lost from the Earth's surface through the combined processes of evaporation and transpiration. a critical parameter in agriculture and irrigation management, as it helps farmers and water resource managers determine the water needs of crops ETa= ET0 * Kc Where: ETc= Actual Evapotranspiration ET0= Reference Evapotranspiration Kc= Crop coeffiecient Difference between Evaporation and Transpiration Transpiration Evaporation Physiological process Physical process It is loss water from the free It is loss water from the free surface of cells surface of water Regulated process Non-regulated process Comparatively slow process It is faster process Influenced by the anatomy of There is no such influence plants Determination of Irrigation Water Requirement Evapotranspiration, ET ETa = ETo × kc Crop Water Requirement, CWR CWR = ETa +(S&P)field Farm Water Requirement, FWR FWR = CWR - ER + LPWR + farm ditch losses Diversion Water Requirement, DWR DWR = FWR +conveyance losses Factors affecting evapotranspiration Weather Parameters Radiation, air, temperature, humidity and wind speed. Crop factors Crop type, variety and development stage Management and environmental conditions Soil salinity, poor land fertility, limited application of fertilizers  Catchment Water Balance Indirect Method Indirect Method ENERGY WATER BALANCE The actual evapotranspiration is estimated by the energy balance λ E= Rn – G - H Where: λE -energy needed to change the phase of water from liquid to gas Rn -Net radiation G- -Soil heat flux H -sensible heat flux Direct Method A lysimeter is a measuring device which can be used to measure the amount of actual evapotranspiration which is released by plants, usually crops or trees. By recording the amount of precipitation that an area receives and the amount lost through the soil, the amount of water lost by evaporation can be calculated ET= P + (I-D)+S Where, ET- Evapotranspiration P- Precipitation I- Irrigation water D- Excess water drained from bottom S- increase or decrease in storage of soil moisture Methods of Estimating Evapotranspiration Methods of Estimating Evapotranspiration Methods of Estimating Evapotranspiration Methods of Estimating Evapotranspiration Example: Determine for the month of June the mean ET0 in mm/day using Blaney-Criddle method. Given: Latitude: 35° North Mean Tmax in June= 29.5 °C Mean Tmin in June= 19.4 °C Formula: ET0= p(0.46T +8.13) Step 3: Calculate ET0: Step 2: Determine p : ET0 = 0.32(0.46*24.5+8.13) Latitude: 35° North ET0 =6.2 mm/day Month: June From Table 4: p=0.32 Thus, the mean reference crop evapotranspiration ET0= 6.2mm/day during the whole month of June. Table 4. Mean Daily percentage of annual daytime hours for different latitudes Latitude North Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec South July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June 60°.15.20.26.32.38.41.40.34.28.22.17.13 55.17.21.26.32.36.39.38.33.28.23.18.16 50.19.23.27.31.34.36.35.32.28.24.20.18 45.20.23.27.30.34.35.34.32.28.24.21.20 40.22.24.27.30.32.34.33.31.28.25.22.21 35.23.25.27.29.31.32.32.30.28.25.23.22 30.24.25.27.29.31.32.31.30.28.26.24.23 25.24.26.27.29.30.31.31.29.28.26.25.24 20.25.26.27.28.29.30.30.29.28.26.25.25 15.26.26.27.28.29.29.29.28.28.27.26.25 10.26.27.27.28.28.29.29.28.28.27.26.26 5.27.27.27.28.28.28.28.28.28.27.27.27 0.27.27.27.27.27.27.27.27.27.27.27.27 Methods of Estimating Evapotranspiration Pan Evaporation Method ET0 = Kp * Epan Where ET0 = reference evapotranspiration, mm/day Kp = pan coefficient Epan = pan evaporation, mm/day Example: Type of pan: Class A evaporation pan water depth in pan on day 1= 150mm water depth in pan on day 2= 144mm (after 24hrs) Rainfall (during 24 hrs)= 0 mm Kpan= 0.75 Formula: ET0 = Kpan * Epan Calculation: Epan = 150-144= 6mm/day ET0 = 0.75* 6= 4.5mm/day Significance of Evapotranspiration Regulates the water cycle Influences climate and weather Affects agriculture and irrigation It maintains soil temperature. It helps in movement of nutrients in plant. It optimizes temperature of plant. Aids drought prediction and monitoring Supports environmental conservation efforts