Computation of excess stormflow at the basin scale. Prof. Pierluigi Claps. Dept. DIATI, Politecnico di Torino

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1 Computation of excess stormflow at the basin scale Prof. Pierluigi Claps Dept. DIATI, Politecnico di Torino losses include: interception, evapotranspiration, storage infiltration, percolation

2 Interception, evapotranspiration, storage First, the falling precipitation may be intercepted by the vegetation or depression storage in an area. It is typically either distributed as runoff or evaporated back to the atmosphere. The leafy matter may also be a form of interception. Interception may be referred to as an abstraction and is accounted for as initial abstraction in some models. Infiltration... Precipitation reaching the ground may infiltrate. This is the process of moving from the atmosphere into the soil. Infiltration may be regarded as either a rate or a total. For example: the soil can infiltrate 1.2 inches/hour. Alternatively, we could say the soil has a total infiltration capacity of 3 inches. Note that in both cases the units are Length or length per time!

3 Percolation... Once the water infiltrates into the ground, the downward movement of water through the soil profile may begin. The percolating water may move as a saturated front - under the influence of gravity Or, it may move as unsaturated flow mostly due to capillary forces. Some Runoff Production Models Phi-Index Psi-Index Bucket Model SCS Curve Number

4 Constant Infiltration Rate Q=P-"# A constant infiltration rate is the most simple of the methods. It is often referred to as a phi-index or!-index. In some modeling situations it is used in a conservative mode. The saturated soil conductivity may be used for the infiltration rate. The obvious weakness is the inability to model changes in infiltration rate. The phi-index may also be estimated from individual storm events by looking at the runoff hydrograph. Hydrograph Breakdown Surface Response Baseflow

5 Hydrograph Breakdown Total Hydrograph Surface Response Baseflow Derive phi-index Flow (cfs) Precipitation (inches) Time (hrs.)

6 Summing Flows > Runoff Volume Continuous process represented with discrete time steps Estimating Excess Precip V = excess precipitation Precipitation (inches) Uniform loss rate of 0.2 inches per hour Time (hrs.) V = runoff volume

7 Phi-Index Summary Phi-index computed as Q-P in volume. Uniform loss rate. If the precipitation stops for a time period, the infiltration will still be the same when the precipitation starts again. Regardless of this weakness, this is still very powerful information to have regarding the response of a watershed. $ index Method Used for medium-scale basins or catchments Estimates the peak discharge (q) from an area q = "! i! A! F where $ is the runoff coefficient (proportional loss) i is the rainfall intensity (mm/h) A is the drainage basin area (km 2 ) F is a units conversion factor = 1/3.6 for [q]=m 3 /s $ has the meaning of the fraction of area that actively contributes to runoff

8 Basic Conceptual Models Bucket model Most simple model Fixed water capacity No soil charaterictics No vegetation Evaporation Water level in bucket Precipitation Runoff Bucket capacity The Biosphere Atmosphere Transfer Scheme (BATS) Two soil layers One vegetation layer Ground Vegetation layer Upper soil layer Root zone layer Total active layer

9 The BATS Model Soil Layer 1 Soil Layer 2 When the layer is full, dump all Soil Layer Soil the extra Starts Layer water filling Keeps into up Filling the next layer with water When the layer is full, dump all Soil the extra Layer water Keeps into Filling the next layer Soil Layer Starts filling up with water And So On... SiB (Simple Biosphere) Two vegetation layers Three soil layers Trees and shrubs Grass Ground Upper thin soil layer Root zone layer Recharge layer

10 Bucket, BATS and SiB models are 1-D models (vertical) Ignore horizontal interactions between adjacent cells Used in 3-D atmospheric models Only three land components (soil, snow and vegetation) No vegetation types No runoff Conceptual Equivalent of the SCS-CN Model P = Precipitation Heavy precipitation causes more runoff than light precipitation S = Storage Capacity Soils with high storage produce less runoff than soils with little storage capacity. F = Current Storage Dry soils produce less runoff than wet soils

11 SCS method Soil conservation service (SCS) method is an experimentally derived method to determine rainfall excess using information about soils, vegetative cover, hydrologic condition and antecedent moisture conditions The method is based on the simple relationship: P e = P - F a I a P e is runoff volume, P is precipitation volume, F a is continuing abstraction, and I a is the sum of initial losses (depression storage, interception, ET) Precipitation Ia t p P = Pe + Ia + Fa Fa P e Time Abstractions SCS Method In general P e! P After runoff begins F a! S Potential runoff P! I a SCS Assumption F S Combining SCS assumption with P=P e +I a +F a P e a P = e P! I a ( P! I ) = P! I a a 2 + S Precipitation Ia t p P = Pe + Ia + Fa Fa P e P = Total Rainfall P = Rainfall Excess I e a a = InitialAbstraction F = Continuing Abstraction Time S = Potential MaximumStorage

12 SCS Method (Cont.) Experiments showed So I a = 0. 2S ( P! 0.2S) P e = P S 1000 S =! 10 CN (American Units; 0 < CN < 100) S = CN! 254 (SIUnits;0 < CN <100) 2 Cumulative Direct Runoff, Pe, in Surface Impervious: CN = 100 Natural: CN < Cumulative Rainfall, P, in SCS Method (Cont.) SCS Curve Numbers depend on soil conditions!.. Group Minimum Infiltration Rate (in/hr) Soil type A High infiltration rates. Deep, well drained sands and gravels B Moderate infiltration rates. Moderately deep, moderately well drained soils with moderately coarse textures (silt, silt loam) C Slow infiltration rates. Soils with layers, or soils with moderately fine textures (clay loams) D Very slow infiltration rates. Clayey soils, high water table, or shallow impervious layer

13 Soil Initial Conditions 5-day antecedent rainfall, inches Antecedent moisture Dormant Season Growing Season I Less than 0.5 Less than 1.4 II 0.5 to to 2.1 III Over 1.1 Over 2.1!!on antecedent rainfall conditions Normal conditions, AMC(II) Dry conditions, AMC(I) Wet conditions, AMC(III) 4.2CN( II ) CN( I) = 10! 0.058CN( II ) 23CN( II ) CN( III) = CN( II )!and on land use, treatment and hydrologic conditions!

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15 The fate of the falling precipitation is: - modeled in order to account for the destiny of the precipitation that falls and to evaluate the precipitation excess that causes the runoff hydrograph.