HYDROLOGY Lecture 3 Evapotranspiration Assoc.Prof. dr.tarkan erdik

A significant part of the precipitation falling from the atmosphere returns back to the atmosphere in form of interception, evaporation and transpiration. Evaporation: is the transformation of water from the liquid state to gas state (water vapor). Evaporation exists when the number of molecules going from the water to the air is larger.

Mechanism of Evaporation 4- Dry air lies above a horizontal water surface with a temperature. 2-Some of the surface molecules have sufficient energy to sever the bonds and enter the thin layer of air just above the surface. The number of molecules with this “escape energy” increases as T increases. The water molecules entering the surface layer move in random motion, and as these molecules accumulate in the layer, some will re-enter the liquid. 1-Water surface:The molecules at the surface are attracted to those in the body of the liquid by hydrogen bonds. 3-An equilibrium is soon reached, at which the rates of escape and re-entry are equal and the vapor pressure in the layer immediately above the surface is equal to the saturation vapor pressure. Water surface

Rate of evaporation is related to various factors. 1- The difference between vapor pressure at the surface (ie saturated vapor pressure of air ew at the temprature of water surface) and vapor pressure of air over water surface ea Dalton’s Law. 2-Air movement: the evaporated water must be carried away from water surface otherwise ea increases and finally ea = ew 3- Energy is required to increase the speed of water molecules to evaporate. Approximately, 539-597 cal of energy is needed to evaporate 1 gram of water. The difference between the two vapor pressures (ew , ea) can be positive, zero, or negative, and a- if ew > ea evaporation is occurring; b- if ew < ea water is condensing on the surface and c- if ew = ea neither condensation or evaporation is occurring.

4- Salts dissolved in water may reduce the evaporation. 5- Evaporation can be reduced by means of a very thin (monomolecular) chemical film formed on the surface of water. 6- Water depth has an important effect on the seasonal rate of evaporation. Deep water masses follow the changes in air temperature with a delay. For this reason, evaporation from deep water is less in summer but more in winter than evaporation from shallow water. 7-Evaporation increases when the air pressure decreases. Annual estimated rate of evaporation is 800 million m3. Dam reservoir

The curve is the relation between saturation vapor pressure and air temperature T Vapor Pressure T Relative humidity At point C, the relative humidty reaches 100% (saturation). This point (C) is called dew point. Further cooling below the dew point will induce condensation of the excess water vapour

E = P+X-Y-F- ∆S Evaporation from Water Surface 1. Water Balance Method Evaporation from water surface (Continuity equation is applied) Daily evaporation rate from the water surface varies between 1 and 10 mm depending on the meteorological conditions. Evaporation rate is important especially important for storage reservoirs. For example, annual estimated rate of evaporation is 800 million m3. Infiltration Difference of water mass in the reservoir Inflow Outflow E = P+X-Y-F- ∆S Evaporation Precipitation Under best conditions 10% error

He = Hi-Ho-Hc-∆H (1) Hc=RHe (2) He=LE (3) 1. Energy Balance Method Energy budget determination (conservation of energy prenciple is applied) Heat loss by conduction Heat outflow Heat required for the change of temprature of water mass He = Hi-Ho-Hc-∆H (1) Conduction is the transfer of heat through a substance between two regions Energy to be used in evaporation Heat inflow Hc=RHe (2) He=LE (3) R:Bowen ratio is usually between 0.2 and 0.3. L: The heat of evaporation for unit volume of water ( 590 cal/cm3 ) E:Volume of evaporated water

3. Masss transfer method 4. Empirical formulas E=C(ew-ea) E:Daily evaporation rate (mm) C:constant. Varies with speed of wind. ew:vapor pressure in water (milibar) ea:vapor pressure in air (milibar)

Mesurement of Evaporation (Class A , the most common type by the U. S Mesurement of Evaporation (Class A , the most common type by the U.S. Weather Bureau) Precipitation? Surface area is 1m2 and water depth of 25cm The pan is filled with water to a depth of 20cm The decrease of water is measured by a point gage, which is an evaporation rate. The pan is placed 15cm above the ground. Everyday water is added so that water surface is 5-8 cm below the upper rim of the pan. Evaporation is the difference between the observed levels It is measured daily. What is pen coefficient? Pan evaporation rates are higher than actual values. That’s why they have to be adjusted with a coefficient as 0.7

Evaporation from soil surface is similiar to that from water surface. If soil is at field capacity, evaporation from soil surface is equal to thatfrom water surface. If soil is at wilting point , evaporation ceases. Ice and snow can directly evaporate from solid phase into liquid phase (sublimation).

Transpiration and interception Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. It may be considered as evaporation from plant tissues. Transpiration occurs during daylight in photosynthesis. Interception is greater at the begining and it decreases as the plants get wetter. Interception might be as much as 1/3 of total precipitation in dense forrests.

Evapotranspiration Losses ET Actual ET Potential ET Potential evapotranspiration (PET) is defined as the amount of ET that would occur if a sufficient water source were available. Actual ET is limited by the actual soil moisture. It might be smaller than PET.

Potantial ET Calculation On annual basis; Coutagne formula Turc formula On monthly basis Hargreaves formula Lowry Johnson formula Blaney-Criddle formula U=45kp(t+18) U: monthly PET (mm) k:coefficient depending on plant (from table) p:ratio of the daytime hours of that month to the total daytime hours of the year (from table) t:monthly mean temprature (0C) On daily basis; Penman formula

k values in Blaney Criddle 100p values

Please apply Thiessen polygons for the basin below (An illustrative case for the previous lecture) Rainfall stations Basin boundary

Rainfall stations Basin boundary

Rainfall stations Basin boundary

Rainfall stations Basin boundary

Rainfall stations Basin boundary