Watershed Modeling PDF

Watershed Modeling By Dr. K. A. Patil (Professor) Civil Engineering Department College of Engineering, Pune Unit II: Watershed Modelling [8 hrs.] Standard modelling approaches and classification, system concept for watershed modelling, overall...

Watershed Modeling By Dr. K. A. Patil (Professor) Civil Engineering Department College of Engineering, Pune Unit II: Watershed Modelling [8 hrs.] Standard modelling approaches and classification, system concept for watershed modelling, overall description of different hydrologic processes, modelling of rainfall runoff process, subsurface flows and groundwater flow.  Watershed Model - Introduction Watershed models - simulate natural processes of the flow of water, sediment, chemicals, nutrients, and microbial organisms & quantify the impact of human activities on these processes. Simulation of these processes plays a fundamental role in addressing a range of 15.00 0.0 watershed based water resources, 12.00 5.0 Rainfall internsity Discharge(m3/sec) 9.00 environmental, social & economical (mm/hr) 10.0 6.00 15.0 problems. 3.00 0.00 20.0 0 200 400 600 Rainfall Time(min) Observed Simulated Watershed Model Main tool in addressing a wide spectrum of environmental and water resources problems – water resources planning, development, design, operation, and management; flooding; droughts; upland erosion; stream bank erosion; coastal erosion; sedimentation; nonpoint source pollution; water pollution from industrial, domestic, agricultural, and energy industry sources; migration of microbes; deterioration of lakes; desertification of land; degradation of land; decay of rivers; irrigation of agricultural lands; conjunctive use of surface and groundwater; reliable design of hydraulic structures and river training works etc.  System Approach System Approach: problems involves following steps; i) Describe the system – involves modeling the watershed system; ii) Describe the objective function - – normally stated in terms of economic terms (eg. Minimize flooding;) Design problems classified: a) Long-run – design of multiple purpose reservoir system – huge capital investment – benefits after & over a long time b) Intermediate run – irrigation & cultivation for a season c) Short- run – how much water to be released for flood control Each require – hydrologic modeling Most situations – alternative models Models: criteria – accuracy, simplicity, consistency & sensitivity  Classification of Models Broadly classified into three types Black Box Models: models describe mathematically the relation between variables (eg. rainfall and surface runoff) without describing the physical process by which they are related. e.g. Unit Hydrograph approach; ANN; Rational formula etc. Lumped models: These models occupy an intermediate position between the distributed models and Black Box Models. e.g. Soil Conservation Curve number method, Stanford Watershed Model Distributed Models: These models are based on complex physical theory, i.e. based on the solution of real governing equation. Eg: Model based on unsteady flow St. Venant equations for watershed modeling. Structure of Watershed Model Simulation of process that takes place in watershed Aim: Gain better understanding of hydrologic phenomena operating in a watershed and how changes in watershed may affect these phenomena Watershed modeling steps: 1. Formulation Rainf Channel phase 2. Calibration/verification all flow 3. Application Watershed model constitutes Infiltration Overland flow 1. Input function 2. Output function 3. Transform function Hydrologic Models - Types Event vs. Continuous models Event model : represents a single runoff event occurring over a period of time ranging from about an hour to several days Accuracy of the model output - Depend on the reliability of initial conditions Continuous watershed model: will determine flow rates and conditions during both, runoff periods and periods of no surface runoff Initial conditions must be known or assumed Utilize runoff components: direct or surface runoff, shallow surface flow (interflow) and groundwater flow An event model may omit one or both of the subsurface components and also evapotranspiration Hydrologic Models – Types… Complete vs. Partial Models Complete or comprehensive watershed models Solves the water balance equation Represents more or less all hydrologic processes Increases the accuracy of the model Partial Models Represents only a part of the overall runoff process Ex: Water yield model gives runoff volumes but no peak discharges  Hydrologic Models – Types… Calibrated Parameter vs. Measured Parameter Models: Calibrated parameter model: One or more parameters that can be evaluated only by fitting computed hydrographs to the observed hydrographs Necessary - If the watershed component has any conceptual component models Period of recorded flow is needed for estimating parameter values Measured parameter model: Determination of parameters from known watershed characteristics Area and channel length – Maps and channel cross sections measured in the field Usually applied to totally ungauged watersheds  Hydrologic Models – Types… Lumped vs. Distributed Models Implicitly Lumped take into models account the spatial variability of inputs, outputs, or parameters Utilize average values of the watershed characteristics affecting runoff lead to significant error- due to nonlinearity and threshold Distributed models values Include spatial variation in inputs, outputs, and parameters. Division of watershed area into a number of elements and calculation of runoff volumes for each element  Hydrologic Models Stochastic Hydrologic models Deterministic Empirical model Lumped Semi distributed Distributed  Precipitation ET Hydrologic Modelin Interception & Storage ET g Surface Surface Runoff – Over land Storage Infiltration Interflow Direct Runoff Percolation GW Base flow Groundwater Channel Flow Flow & & Processes Storage Direct runoff Flowchart of simple watershed model (Based on: McCuen, 1989)  Watershed Simulation Analysis Model selection criteria Assumptions & conceptualization Ability of model to predict variables required by the project Hydrologic processes that need to be modeled to estimate the desired outputs adequately (single-event or continuous processes) Availability of input data Expertise available & computational facility Price Surface detention Watershed Simulation Analysis Steps: Selection of model Input data collection: rainfall, infiltration, physiography, land use, channel characteristics etc Evaluate the study objectives under various watershed simulation conditions Selection of methods for obtaining basin hydrographs and channel routing Calibration and verification of model Model simulations for various conditions Sensitivity analysis Evaluate usefulness of model and comment on needed changes.  Estimation of Surface Runoff Empirical Equations: Rational method (Empirical model) Q=CiA where, q = design peak runoff rate m3/s; C = Runoff coeffi cient; I = rainfall intensity (mm/hr) for design return period and for a duration equal to time of concentration of the watershed; A = watershed area in ha; C = Runoff coeffi cient (rate of peak runoff rate to rainfall intensity, (dimensionless)); C varies as per slope, land use etc. e.g. available -- 0.3 to 0.6 (0-5 % slope); - 0.1 to 0.3(0-5 % slope). C1 A1  C2 A2  C3 A3 Watershed of different characteristics: C A A  A1  A2  A3 To help protect your priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content. 15.00 0.0 12.00 Discharge(m3/sec) 5.0 Rainfall internsity 9.00 (mm/hr) 10.0 6.00 15.0 3.00 0.00 20.0 Rational 0 200 400 Time(min) 600 Rainfall Observed Simulated Method Assumptions: Rainfall occurs at uniform intensity, Tc- equal to the time of concentration of watershed Rainfall occurs at the uniform intensity over whole area Max. runoff is directly proportional to rainfall intensity Peak discharge probability is same as rainfall probability Runoff coeffi cient does not change with storm type Time of concentration: It is the time needed for water to flow from the most hydrological distant point in the watershed to the outlet once the soil has become saturated and minor depressions filled When duration of rainfall storm equals time of concentration, all parts of watershed contribute simultaneously to the runoff at the outlet  Rational Method… Kirpich (1940) formula for Tc where, Tc- in minutes Tc  0.0195L0.77 S g 0.385 L- Max. length of flow in m Sg- Watershed gradient in m/m (difference between outlet and most remote point divided by length L Modified Kirpich equation: Where, Lo- Length of overland flow in m 0.467 S0- Slope along path in m/m Tc  0.0195L0.77 S g 0.385   2Lon   S  n- Manning’s roughness coeffi cient Eg: Poor grass, cultivated raw crops n=0.2; smooth  impervious n=0.02 Rational method limited to area less than o  800 Ha 18 Other Empirical Equations Dicken’s formula: Q  CA0.75 where, Q Peak rate of surface runoff in m3/s A Area in km2; C Coeffi cient e.g. 11.45 for annual rainfall : 610 to 1270 mm Ryve’s formula: Q  CA0.67 Same Q and A and C varies from 6.76 to 40.5 depending on location of watershed (suitable for South India) Based on practical experience and long term observations Other Empirical Equations Cook’s method – Evaluated by relief, soil infiltration, vegetation cover and surface storage – Approximate weightage are aligned for those parameters QPRF S Where, Q Peak runoff for specific region; P Peak runoff from groups; R Geographic rainfall factor from groups F Return period from groups; S shape factor from Table  Soil Conservation Service (SCS) Method(1999) Evolved for uniform rainfall using assumptions for a triangular hydrograph; Assumptions of rational method and corresponding SCS triangular hydrograph D D Time for peak flow Tp  2  T L  2  0.6T c Tp- Time of peak; D- Duration of excess rainfall; TL- Time of Lag; Tc- Time of 0.7 Concentration; Tc=TL /0.6 L-   1000  0.8  L 9  Longest flow length in m. N- Tc    N    Runoff curve number and 4407S  g 0.5 Sg- average watershed gradient in m/m  SCS Method Peak flow rate(m3/s) q  qn AQ qn= unit peak flow rate (m3/sec per ha/mm of runoff) A-Watershed area in ha Q-Runoff depth in mm from curve number method Unit peak flow rates are developed for a particular region using time of concentration and ratio of initial abstraction to 24 hour rainfall Curve Number Method Developed based on observation in agricultural watersheds in USA for long time—rainfall and runoff by USDA. SCS-CN method –NRCS (Natural Resources Conservation Service) Based on recharge capacity of a watershed Recharge capacity based on antecedent moisture content and physical characteristics of watershed Curve Number is an index that represents combination of a hydrologic soil group and antecedent moisture conditions  SCS-CN Method Hydrologic Soil Groups: SCS(1972) Group A (Low runoff potential):-Soil with high infiltration rates when thoroughly wetted, consisting mainly of deep well to excessively drained sands and gravels – High rate of transmission Group B (Moderately low runoff potential) – Moderate infiltration rates:-moderate rate of water transmission Group C (Moderately high runoff potential) – Slow infiltration rate Group D (High runoff potential) –slow infiltration Eg. Clay pan or layer SCS-CN Method Antecedent Moisture Condition (AMC) Index of watershed wetness which is determined by total runoff in 5 days period preceding a storm AMC I – Lowest runoff potential soil dry enough for cultivation AMC II – Average condition AMC III – Highest runoff potential practically saturated  SCS-CN Method…  Potential maximum retention storage of watershed is related to curve number (Dimensionless); 0 to 100  Let Ia is the initial amount of abstractions (interception, depression storage & infiltration). It is assumed that ratio of direct runoff Q and rainfall P minus initial loss (P-Ia) is equal to ratio of actual retention to storage capacity, S Q P  Q  Ia (1)  P  Ia S where, Ia -Initial amount of abstraction; Ia is assumed to be a fraction of S on an average Ia=0.2S  SCS-CN Method… Therefore, Equation (1) becomes (P  0.2S ) 2 Q P  0.8S Knowing P and S, value of Q can be Computed. Q has same units as P (in mm) For convenience in evaluating antecedent rainfall, soil conditions and land use practices, curve number CN  25400 254 S where, S- recharge capacity of watershed Runoff SCS-CN Method- Runoff CN for different hydrologic soil Cover Hydrologic Groups Land Treatme Use or nt or Hydrologic A B C D cover practice condition 1 2 3 4 5 6 7 Fallow Straight ---- 77 86 91 94 row Row Straight Poor 72 81 88 91 crops row Row Straight Good 67 78 85 89 crops row 2 8 2 8 SCS-CN Method… Above Table, given for antecedent rainfall condition II ; Average condition Heterogeneous watershed may be divided into sub areas with different numbers and then a weighted curve number can be computed for those watershed Various tables available for various conditions Also CN can be developed for local conditions depending upon soil & other conditions. SCS-CN Method… Relationship between rainfall & runoff curve number CN (after Soil Conservation Service 1972) Example Problem Calculate the runoff from a watershed of 50 Ha for the following data using SCS-CN method. Depth of rainfall=150mm; Antecedent Moisture condition, AMC I. Row crop, good condition in 30 Ha; Woodland, good condition in 20Ha. Type of crop CN at AMCII AMCI Row crop, good 82 82x0.8= 65.6 Woodland, good 55 55x0.65= Weighted CN = (65.6 x 30 + 35.75 x 20)/35.75 50 = 53.66 Using CN  25400 ; S= 219.35 254  S (P  0.2S ) 2 Q= 34.606 mm; Runoff in response to Q P  0.8S 150mm rainfall Prof. T I Eldho, Department of Civil Engineering, IIT Bombay 31 THANK YOU