Precipitation P: water (solid, liquid) falling from atmosphere to ground P includes Rain, Drizzle, Snow, Hail, Sleet, Ice crystals Measurement: Container to collect P in a storm event Radar, digital recorders Accuracy depends on physical setting, disturbances…etc Weather data available from government agencies: PME (MEPA), MWE (MAW), …

Formation Of Precipitation Conditions: Humid air cooled to dew-point T Nuclei Droplets to raindrops Size of raindrops

Standard Rain Gages (SRG) 8 in (20.32 cm) Standard Rain Gages (SRG) 30 in (76.2 cm)

Effective Depth Of Precipitation (UED) Arithmetic average: for evenly distribute stations (uniform density) Thiessan method area-weighted averaging Isohyetal lines contouring

Areal Estimation of P from a network of gages 13.97 mm 22.1 mm 137.2 mm 59.2 mm 48 mm

(1) Arithmetic average Pa = 1/N ∑ Pi (13.97 + 22.1 + 59.2 + 48.0+ 137.2)/5 = 56.1

(2) Thiessan Polygon Method Area-weighted average (every gage represents best the area immediately around the gage) Constructing Thiessan Network: Plot stations on a map Connect adjacent stations by straight lines Bisect each connecting line perpendicularly Perpendicular lines define a polygon around each station P at a station is applied to the polygon closest to it

Weight-ed P Weighted area Polygon area P St.No 1.788 0.128 15 13.97 1 9.273 0.281 33 22.1 2 14.5 0.245 28.8 59.2 3 6.672 0.139 16.4 48 4 28.4 0.207 24.3 137.2 5 60.633 1.00 117.5 280.47 Totals

)3) isohyetal method Based on areas calculated from contoured P map (check first for effect of elevation by plotting P vs elevation) STEPS: Plot a contour map of P based on gage readings at station Compute area between each successive contour lines Pa = PaiAi/  Ai

Isohyetal method

Isohyetal method procedure Determine contours of equal P: (Isohyetal lines) Estimate representative P for each region Calculate Pav P = Pi*Ai/AT = P(1)*A(1)/AT + P(2)*A(2)/AT + P(3)*A(3)/AT +P(4)*A(4)/AT 30 25 20 28 26 15 (4) 22 (3) 17 (2) (1) 11

Events During Precipitation Interception (8- 35% for densely vegetated) Stem flow Infiltration infiltration capacity how fast water is absorbed into soil.  effected greatly by soil type.  Water infiltrates faster into sand than it does in clay  Low infiltration capacity causes more runoff and more erosion.

Depression storage Overland flow (P rate > F rate) Interflow (horizontal flow in unsaturated zone) Baseflow

Evaporation It’s the physical process by which liquid is transformed to gas (vapor) due to the release of the bonds holding the molecules together In Hydrology: it’s the amount of water lost from soils and open water bodies to the atmosphere Evaporation stops when air is saturate with moisture Absolute humidity: Grams of water/cubic meter of air Saturation humidity max amount of moisture air can hold at any T Relative humidity: Absolute humidity/saturation humidity

Standard: US weather service class A Pan Evaporation contd. Dew point: T at which condensation will begin Rate of E depends on: T (air), T(water), absolute humidity, wind. Estimation and Measurement: No direct measurement Water budget method Eo = I - O (+/-) S Evaporation pan Standard: US weather service class A Pan

Evaporation estimation 4 ft (1.22 m) 10” (25.4 cm)

Evaporation Pan operation: Pans placed on supports to allow air circulation A water depth of 7-8 in (18 - 20 cm) is maintained. Water depth in pan is measured with time Max, min, T recorded Water is added or removed from pan to adjust for rain and E, its volume recorded E from a pan is higher than actual E from a lake Pan coefficient: Empirical correction factor 0.58 – 0.78 (depending on month)

Transpiration Mass transfer of water from ground to the air through plants Transpiration can exceed evaporation in heavily wooded areas Transpiration is only important during growing season (in cultivated areas) wilting point: soil moisture is low causing surface tension of soil-water interface to exceed osmotic pressure  water will not enter roots Transpiration is measured in carefully controlled lab conditions Phytometer: is a sealed container partially filled w/ soil. Transpiration is measured as the increase in humidity in the air space around the plant

Evapotranspiration (ET) any transfer of moisture to the air 90% of P in arid regions! In field conditions, not possible to separate evaporation from transpiration Potential ET: maximum evapotranspiration if there is infinite supply of water available in the soil for the vegetation. Actual ET: amount of ET under field conditions

Potential ET

Theoretical estimation (empirical formulas) Thornthwaite's method Blaney and Criddle method Penman Method field measurement Neutron-Probe method (most accurate method) Lysimeter Large watertight caisson buried in ground, filled w/ soil and planted with vegetation.

Thornthwaite Eqn.

Infiltration, Overland Flow, and Interflow

Infiltration, Overland Flow, and Interflow

Infiltration, Overland Flow, and Interflow Infiltration: process of downward entry of water into soil Infiltration rate depends on: Sensitive to soil condition, Antecedent water content of soil Influenced by soil use and management Decreases with increasing time Overland flow = surface runoff: flow across land surface to channels Inflow: lateral flow of water above water table

Infiltration, Overland Flow, and Interflow

Infiltration, Overland Flow, and Interflow Infiltration: process of downward entry of water into soil Infiltration rate depends on: Sensitive to soil condition, Antecedent water content of soil Influenced by soil use and management Decreases with increasing time Overland flow = surface runoff: flow across land surface to channels Inflow: lateral flow of water above water table