Watershed Hydrology NREM 691 Week 3 Ali Fares, Ph.D. Evapotranspiration Watershed Hydrology NREM 691 Week 3 Ali Fares, Ph.D. 11/28/2018 Watershed Hydrology Lab. Fall 2006

Objectives of this chapter Explain and differentiate among the processes of evaporation from a water body, evaporation from soil, and transpiration from a plant Understand and be able to solve for evapotranspiration (ET) using a water budget & energy budget method Explain potential ET and actual ET relationships in the field. Under what conditions are they similar? Under what conditions are they different? Understand and explain how changes in vegetative cover affect ET. Describe methods used in estimating potential and actual ET 11/28/2018 Watershed Hydrology Lab. Fall 2006

Watershed Hydrology Lab. Fall 2006 Energy Budget L is latent heat of vaporization, E evaporation, H energy flux that heats the air or sensible heat, G is heat of conduction to ground and Ps is energy of photosynthesis. LE represents energy available for evaporating water Rn is the primary source for ET & snow melt. Net radiation: Rn=(Ws+ws)(1- α)+Ia-Ig Rn is determined by measuring incoming & outgoing short- & long-wave rad. over a surface. Rn can – or + If Rn > 0 then can be allocated at a surface as follows: Rn = (L)(E) + H + G + Ps 11/28/2018 Watershed Hydrology Lab. Fall 2006

Watershed Hydrology Lab. Fall 2006 An island of tall forest vegetation presents more surface area than an low-growing vegetation does (e.g. grass). The total latent heat flux is determined by: LE = Rn + H Advection is movement of warm air to cooler plant-soil-water surfaces. Convection is the vertical component of sensible-heat transfer. In a watershed Rn, (LE) latent heat and sensible heat (H) are of interest. Sensible heat can be substantial in a watershed, Oasis effect were a well-watered plant community can receive large amounts of sensible heat from the surrounding dry, hot desert. See Table 3.2 comparison See box 3.1 illustrates the energy budget calculations for an oasis condition. 11/28/2018 Watershed Hydrology Lab. Fall 2006

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ET & Available Soil Water 11/28/2018 Watershed Hydrology Lab. Fall 2006

Water movement in plants Illustration of the energy differentials which drive the water movement from the soil, into the roots, up the stalk, into the leaves and out into the atmosphere. The water moves from a less negative soil moisture tension to a more negative tension in the atmosphere. 11/28/2018 Watershed Hydrology Lab. Fall 2006

Watershed Hydrology Lab. Fall 2006 Yw~ -1.3 MPa Yw~ -1.0 MPa Yw~ -0.8 MPa Yw~ -0.75 MPa Yw~ -0.15 MPa Ys~ -0.025 MPa 11/28/2018 Watershed Hydrology Lab. Fall 2006

Measuring ET: Pan-Evaporation 11/28/2018 Watershed Hydrology Lab. Fall 2006

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Watershed Hydrology Lab. Fall 2006 PET Definitions Penman Definition: The amount of water transpired in unit time by a short green crop completely shading the ground, of uniform height and never short of water. Therefore, PET based on atmospheric conditions and a specific vegetation type 11/28/2018 Watershed Hydrology Lab. Fall 2006

Methods to Calculate PET Empirical Estimation Equations for a Reference Crop Kimberly – Penman Combination Equation Combines energy and diffusion components Radiation based equations Priestley – Taylor equation Turc equations Doorenbos and Pruitt (FAO-24) method Jensen – Haise method 11/28/2018 Watershed Hydrology Lab. Fall 2006

Methods to Calculate PET Physically Based Equation Penman-Monteith Reference Crop Evaporation Equation The most advanced model of evaporation Assumes all energy available is accessible by the crop canopy Uses: Net radiation (Rn) Soil heat flux (G) Air temperature Wind speed (U) Vapor pressure deficit 11/28/2018 Watershed Hydrology Lab. Fall 2006

Methods to Calculate PET Empirical Estimation Equations for a Reference Crop Temperature based equations Hargreaves equation Blaney – Criddle (SCS TR-21) method Thornthwaite method Pan Evaporation Methods FAO-24 method Christiansen – Hargreaves method 11/28/2018 Watershed Hydrology Lab. Fall 2006

Water Mass balance Equation S =(I + R + U) - (D + RO + ET) ET = Evapotranspiration R, I = Rain & Irrigation D = Drainage Below Rootzone RO = Runoff S = Soil Water Storage variation U = upward capillary flow 11/28/2018 Watershed Hydrology Lab. Fall 2006

Watershed Hydrology Lab. Fall 2006 Evapotranspiration ET = P – Q – ΔS - ΔD ΔS= watershed storage variation (mm): Send–Sbeginning P = Precipitation (mm) Q = Stream flow (mm) ΔD = Seepage out – seepage in (mm) ET = evaporation and transpiration (mm) This equation is telling us that if you know the water content in the soil profile now (St) and after a certain period of time (1 hour, 1 day) and if you also know how much irrigation or rainfall that was added into the system you can determine the losses out of the system D and ET. Thus, this equation has two unknowns: ET and D. If we determine one of them then the equation will be left with only one unknown that can be determined form the equation. ET can be determined from weather data and using mathematical equation such as penman-monteith. It can be determined form lysimeter measurements. For our case we choose to calculate drainage using Darcy’s flow equation. 11/28/2018 Watershed Hydrology Lab. Fall 2006

Soil Water Mass Balance There are different ways to estimate drainage. The direct method is the use of lysimeters. Lysimeters have a weighing device and a drainage system, which permit continuous measurement of excess water and draining below the root zone and plant water use, evapotranspiration. Lysimeters have high cost and may not provide a reliable measurement of the field water balance. 11/28/2018 Watershed Hydrology Lab. Fall 2006

Watershed Hydrology Lab. Fall 2006 Rain Transpiration Evapo-transpiration Irrigation Evaporation Runoff Root Zone Water Storage Below Root Zone Drainage 11/28/2018 Watershed Hydrology Lab. Fall 2006

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Effects of Vegetative Cover 11/28/2018 Watershed Hydrology Lab. Fall 2006

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Watershed Hydrology Lab. Fall 2006 ET / Potential ET 11/28/2018 Watershed Hydrology Lab. Fall 2006