HYDROLOGIC ABSTRACTIONS Problems in hydrologic design requires the modelling of precipitation-runoff relationship.  Runoff = Total Precipitation – “ Losses ” Types of Hydrologic Abstractions (Losses): (A)  Interception; (B) Depression Storage; (C)  Evaporation; (D)  Transpiration; (E) Evapotranspiration; (F) Infiltration.

Interception Depression Storage Definition: Fraction of the gross precipitation input which wets and adheres to above ground objects until it is returned to the atmosphere through evaporation. Depression Storage Definition: Rainwater retained in puddles, ditches, and other depressions on the ground surface. As soon as rainfall intensity exceeds the local infiltration capacity, the rainfall excess begins to fill depression.  Water held in depression at the end of rain either evaporates or contributes to soil moisture and/or subsurface flow by following the infiltration. 

Evaporation Definition: Process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat energy . Evaporation is dependant on following factors: (a)  Solar radiation; (b) Vapour pressure at water surface and the overlaying air; (c)  Temperature; (d) Wind; (e)  Atmospheric pressure; (f)  Quality of water; (g) Nature and size of evaporating surface;  

Factors affecting evaporation losses: Nature of evaporating surface Area of water surface Depth of water in water body Humidity Wind velocity Temperature Atmospheric pressure quality of water

Evaporation Determination 1) By Using Evaporation Pans: It is most commonly used method for determining the evaporation. Etrue = Cp x Epan   where Cp is pan coefficient, 0.70~0.95. Value of Cp varies considerable from month to month, but fairly consistent from year to year.   Types of Pan : a) Class A Evaporation Pan b) ISI standard pan c) Colorado Square Sunken Pan d) US Geological survey Floating Pan

Evaporation Pans

2) Estimation of evaporation by empirical equations- There are large number of equations for estimating the evaporation loss. Meyer’s Formula (1915) Rohwer’s formula (1931) 3) Water Budget or Water Balance method- This method balances all the incoming, outgoing and stored water in a lake or reservoir over a period of time, using following equation- ∑ Inflow - ∑Outflow = Change in storage + Evaporation loss E = ∑I - ∑O ± ∆S

Water Budget or Water Balance method- Precipitation - P Evaporation- E Inflow- Q Surface runoff - Qr Subsurface runoff - Qs Subsurface seepage losses- Qd Outflow- Q0

4) Energy budget or energy balance method- Total solar radiation - Rt Net energy advected (net energy content of incoming and outcoming water - Ee Reflected solar radiation - Rr Net long-wave radiation exchange between the atmospere and the water body- R1 Sensible heat loss from the water body to the atmosphere - Hn Energy used for evaporation (latent heat)- Ee Energy stored - Es R1 includes long-wave (LW) radiation from the atmosphere, reflected LW radiation, LW radiation emitted by water

Transpiration Factors Affecting Transpiration A process whereby water is taken from the soil moisture storage by roots and passes through the plant structure and is evaporated from cells in the leaf called stomata. Factors Affecting Transpiration a) Temperature; b) Solar Radiation; c) Wind; d) Soil Moisture; e)  Plant type Measurement of Transpiration  a. Phytometer b. Potometer  

Evapotranspiration Evapotranspiration (ET) = Consumptive Use The process by which water is evaporated from wet surface and transpired from plants, i.e. sum of evaporation and transpiration. Evapotranspiration (ET) = Consumptive Use Potential Evapotranspiration (PET) The evapotranspiration (ET) that would occur if there was an adequate soil-moisture supply at all time. Actual Evapotranspiration (AET) The real evapotranspiration occur in a specific situation is called actual evapotranspiration.

Determination of ET Direct measurement of consumptive use or evapotranspiration- (a) Tank and Lysimeter Method (b) Field Expiremental Plots (c) Soil Measurement Studies (d) Integration Method (e) Inflow Outflow Studies   ET Equations Due to the lack of basic data and the difficulties in measurement required in the field methods lead to the development of ET equations that relates the ET with readily available climatic data. Blaney-Criddle Method Penmon Method Hargreaves class A Pan Evaporation Method

Blaney-Criddle Method- Cu = k x f Where, Cu - monthly consumptive use in ‘cm’ k - monthly crop coefficient f - monthly consumptive use factor Penmon Method- Et = Where, Et – Evapotranspiration ‘mm/day’ A – mean air temperature α – constant H – daily net radiation in ‘mm’ Ea - drying power of air which includes wind velocity.

Hargreaves class A Pan Evaporation Method – Et or Cu = K x Ep Where, K – consumptive use coefficient Ep – class A pan evaporation Cu – consumtive use or evapotranspiration

Infiltration · Infiltration may be defined as the downward movement of water from soil surface, into the soil mass through the pores of soil. When rain water falls on the ground , a small part of it is initially absorbed by the top thin layer of soil so as to replenish the soil moisture deficiency, & thereafter, excess water infiltrates downwards to join ground water. OR In simple term the entry or passage of water into the soil through soil surface is called as infiltration.   ·  Factors Affecting Infiltration Rate: (a)  Type & extent of vegetal cover; (b) Condition of surface crust; (c)  Temperature; (d) Rainfall intensity; (e)  Soil properties; (f) Water quality.

Factors Affecting Infiltration Rate

Field Measurement of Infiltration Rate- Infiltration in the field can be measured with the help of two types of infiltrometers. Single cylindrical or single tube infiltrometer Concentric double cylindrical or double ring infiltrometer Types of infiltration- Ф Index W Index Wmin Index

Measurement of Infiltration...... Infiltrometer: An artificial application of water to enclosed sample areas. (see Figure in handout) Types: (a.1) Rainfall Simulator - Infiltration capacity is determined from rainfall-runoff hydrographs. (a.2) Flooding Type – Include tubes & concentric rings. Accumulate Rainfall = Accumulate Infiltration = Volume of H2O added / area of ring (b) Hydrograph Analysis: Analyse rainfall-runoff hydrographs that are actually occurring in a watershed. The derived estimation of infiltration can ultimately be no more accurate than the precision of the measurements of rainfall & runoff.

Infiltration Terminologies Infiltration Capacity (Potential Infiltration Rate) (fp) - The maximum rate at which soil can absorb water through its surface. Infiltration Rate, f(t) - Rate of water entering the soil surface. If there is no limit on the water supply for infiltration, f(t) = fp. Otherwise, 0  f(t) < fp. fo = initial infiltration rate fc = ultimate infiltration rate f(t) - fc = excess infiltration rate   Cumulative Infiltration, F(t) - Depth of infiltration from the beginning of rainfall to any time, t. F(t) = Area under the infiltration curve Wetting Front - Change of soil moisture content with depth is so great so as to give the appearance of a sharp discontinuity between the wet soil above and the dry soil below. f(t) fo fc t

Runoff- Runoff is defined as the portion of ppt which is not absorbed by deep strata but finds its way into the stream after meeting the resistance of evapotranspiration including interception and other losses. Overland Flow – Consider catchment area receiving ppt. For a given ppt, ET, initial losses, infiltration and detention storage requirement will have to be first satisfied before the catchment of runoff. When these are satisfied, the excess ppt move over the land surface to reach small channels. This portion of runoff is called as overland flow. Surface Runoff – Flow from several small channel joints, bigger channel and flow these intern combine to form a large stream and so on, till the flow reaches the catchment outlet. The flow in this mode where it travels all the time over the surface as overland flow and through the channels as open channel flow and reaches the catchment outlet is called surface runoff.

Factors affecting Runoff- Nature of Precipitation a. Rainfall Intensity b. Frequency of intense rainfall c. Duration of Rainfall d. Distribution of rainfall Physical Characteristics of the drainage basin a. Size of catchment b. Shape of catchment c. Slope of catchment d. Orientation of catchment Meteorological Characteristics of the basin Geographic features of the basin Storage characteristics of the basin Other factors

Computation of Rainfall- By linear or exponental regression By empirical equations and tables By Infiltration method a) ф – index b) W – index c) Wmin - index By Unit Hydrograph By Rational method