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INTERCEPTION, DEPRESSIONSTORAGE AND INFILTRATION
Interception – loss of rainfall due to vegetation (fromtrees to grass) rainfall throughfall stemflowInterception = f (vegetation (age, density, type), season, rain intensity, antecedentconditions)Forest cover data collected by placing rain gauges under forest canopy andcomparing with gauge data from open area.
Factors affecting interception• Storm characteristics The number and spacing of precipitation events, intensity and amount of precipitation Wind speed• Vegetation characteristics Species, age, density, and condition of vegetation
Estimation of interceptionLi = Si + KEtLi = volume of water intercepted (inches)Si = interception storage that will be retained on the foliage, (0.01 to 0.05 in) f (wind, gravity, type)K = ration of surface area of intercepting leaves to horizontal projection of tree area, Light storms K = 100% Heavy storms K = 10 to 40 %E = amount of water evaporating per hour during precipitation (in/hr)t = time, hrs
DEPRESSION STORAGE• Depression storage, or ponding, is that water on a drainage a drainage basin that drains into closed depressions and never reaches the outlet of the basin.• This water becomes trapped in ponds; some eventually evaporates and the remainder infiltrates into the ground.• Depression storage occurs on most basins.
INFILTRATION• Infiltration is the vertical movement of water through the soil surface.• Similar terminology: percolation into the soil seepage out of the soil
• Infiltration is highly related to soil properties.• Our ability to physically represent the infiltration process is related to being able to represent soil properties.• However, soils exhibit a great deal of variability spatially and vertically.• Thus our representations generalize over a large variability of soil characteristics
Process of infiltration, Moisture content soil moisture descriptionSoil water zone – max depth from whichwater can be returned to surface through Saturated zonecapillary action or ET. Transmission zone,Gravitational water – flow direction uniform moistureis vertical due to gravity. content, not saturated(unsaturated zone or zone ofaeration) Capillary zone, less than atmospheric pressure Wetting front Groundwater, saturation at atmospheric pressure
Moisture Content:Total volume = air volume (voids) + solid volumeη, porosity = volume of voids ÷ total volumeθ, soil moisture content = volume water ÷ total volumemaximum θ = η
Measurement:Initial efforts to describe infiltration are based on measured data.Split (double) ring infiltrometer • shown to represent Horton parameters fairly well. • Measure rate of vertical movement from center ring • Exterior ring to offset lateral movement of moisture • Change in elevation measured at selected time intervals (commonly use a point gauge). • Actually measures maximum infiltration capacity because excess water is available. 35 cm 23 cm
Sprinkler infiltrometercatch and measure runoff rateinfiltration rate = “rainfall” rate - runoff rate12’6’usually have high application rate therefore approaching maximum infiltration rate
RUNOFFRunoff or overland flow will occur if the amount ofwater falling on the ground is greater than theinfiltration rate of the surface,. Runoff specifically refers to the water leaving anarea of drainage and flowing across the land surfaceto points of lower elevation.It is not the water flowing beneath the surface of theground.This type of water flow is called throughflow.
Runoff involves the following events:• Rainfall intensity exceeds the soils infiltration rate.• A thin water layer forms that begins to move because of the influence of slope and gravity.• Flowing water accumulates in depressions.• Depressions overflow and form small rills.• Rills merge to form larger streams and rivers.• Streams and rivers then flow into lakes or oceans.
Runoff on a global scale • Surface runoff sends 7 % of the land based precipitation back to the ocean to balance the processes of evaporation and precipitation.Continent Runoff Per Unit Area (mm per yr.)Europe 300Asia 286Africa 136North and Central America 265South America 445Australia, N.Zealand and New 218GuineaAntarctica and Greenland 165
Streamflow and Stream Discharge• The term streamflow describes the process of water flowing in the organized channels of a stream or river.• Stream discharge represents the volume of water passing through a river channel during a certain period of time.• Stream discharge can be expressed mathematically with the following equation: Q=WxDxV – where, – Q equals stream discharge usually measured in cubic meters per second, W equals channel width, D equals channel depth, and V equals velocity of flowing water.
Stream hydrograph• Because of streamflows potential hazard to humans many streams are gauged by mechanical recorders. These instruments record the streams discharge on a hydrograph.
From this graph we can observe the following things:A small blip caused by rain falling directly into the channel is the first evidencethat stream discharge is changing because of the rainfall.A significant time interval occurs between the start of rain and the beginning ofthe main rise in discharge on the hydrograph. This lag occurs because of the timerequired for the precipitation that falls in the streams basin to eventually reach therecording station. Usually, the larger the basin the greater the the time lag.The rapid movement of surface runoff into the streams channels andsubsequent flow causes the discharge to rise quickly.The falling limb of the hydrograph tends to be less steep that the rise. This flowrepresents the water added from distant tributaries and from throughflow thatoccurs in surface soils and sediments.After some time the hydrograph settles at a constant level known as base flowstage. Most of the base flow comes from groundwater flow which moves waterinto the stream channel very slowly.
the shape and magnitude of the hydrograph is controlled by two sets of factors:Permanent Factors - slope of basin, soil structure, vegetation, channel density,etc.Transient Factors - are those factors associated with precipitation input - size ofstorm, intensity, duration of rainfall, etc.
Runoff ModelsHistorical PerspectiveThe development and application of hydrological models have gone through a long time period, the remarkable dates in the history of the development of hydrological models are:The origins of rainfall-runoff modelling in the broad sense can be found in the middle of the 19th century, when Mulvaney (1850), an Irish engineer who used in the first time the rational equation to give the peak flow Qp as: Qp = CiA Where,C is the coefficient of runoff (dependent on catchment characteristics)i is the intensity of rainfall in time Tc andA is the area of catchment.Tc is the time of concentration, the time required for rain falling at the farthest point of the catchment to flow to the measuring point of the river.
A major step forward in hydrological analysis was the concept of the unithydrograph introduced by the American engineer Sherman in 1932 on the basisof superposition principle.The use of unit hydrograph made it possible to calculate not only the flood peakdischarge (as the rational method does) but also the whole hydrograph (thevolume of surface runoff produced by the rainfall event).The real breakthrough came in the 1950s (Todini, 1988) when hydrologistsbecame aware of system engineering approaches used for the analysis ofcomplex dynamic systems. This was the period when conceptual linear modelsoriginated (Nash, 1958, 1960).Many other approaches to rainfall-runoff modelling were considered in the 1960s.A large number of conceptual, lumped, rainfall-runoff models appeared thereafterincluding the famous Stanford Model IV (Crawford and Linsley, 1966) and theHBV model (Bergström and Forsman, 1973). Stochastic time series models were first introduced by Box and Jenkins (1970) which provided hydrologists with an alternative model type.
One remarkable model developed in the late 1970s is the TOPMODEL (Beven andKirkby, 1979) that is based on the idea that topography exerts a dominant control onflow routing through upland catchments is called.To meet the need of forecasting (1) the effects of land-use changes, (2) the effectsof spatially variable inputs and outputs, (3) the movements of pollutants andsediments, and (4) the hydrological response of ungauged catchments where nodata are available for calibration of a lumped model, the physically-baseddistributed-parameter models were developed. The Systéme HydrologiqueEuropéen (SHE) model is a excellent example of such models (Abbott et al., 1986).
The macro-scale hydrological models were developed on the basis of the following motivations. 1. First, for a variety of operational and planning purposes, water resource managers responsible for large regions need to estimate the spatial variability of resources over large areas, at a spatial resolution finer than can be provided by observed data alone. 2. Second, hydrologists and water managers are interested in the effects of land-use and climate variability and change over a large geographic domain. 3. Third, there is an increasing need of using hydrologic models as a base to estimate point and non-point sources of pollution loading to streams. 4. Fourth, hydrologists and atmospheric modellers have perceived weaknesses in the representation of hydrological processes in regional and global atmospheric models. 5. Examples of GIS supported macro-scale hydrological models include those developed by Vörösmarty et al. (1989), the VIC model (Wood et al., 1992) and the Macro-PDM (Arnell, 1999). These models are state-of-the- art tools in assessing regional and continental scale water resources.
Applications of hydrologic modelsNowadays, mathematical models have taken over the most important tasks inproblem solving in hydrology. The important applications of hydrological model aresummarised below:Design Operation Dams and reservoirs Flow forecasting design Reservoir control water yield Urban storm drain control capacity, failure Management Floods Land-use changes frequency Climate changes mapping Point/nonpoint pollution Urbanisation Groundwater recharge storm drains Research and teaching flood plains University training channel alterations Industrial trainingIrrigation and drainage Research
Runoff models are probably what most hydrologists spontaneously refer to whendiscussing hydrological models.This was also the first branch in which models were used when computersbecame easily available in the 1970s.The basic principle in hydrological modelling is that the model is used to calculateriver flow based on meteorological data, which are available in a basin or in itsvicinity.Hydrological models include subroutines for the most significant hydrologicalprocesses, such as snow accumulation and melt at different elevations, soilmoisture dynamics, evapotranspiration,recharge of groundwater, runoffgeneration and routing in lakes and rivers.Most runoff models are based on the water balance, using precipitation as adriving variable and calculating the quantities directed as runoff, R, from the waterbalance equation,R = P – E – DS,where P is precipitation, E evapotranspiration, and DS represents various storageterms.
Runoff and Hydrologic Modeling (RS) Runoff cannot be directly measured by remote sensing techniques. However, there are two general areas where remote sensing can be used in hydrologic and runoff modeling:4. determining watershed geometry, drainage network, and other map-type information for distributed hydrologic models and for empirical flood peak, annual runoff or low flow equations; and5. providing input data such as soil moisture or delineated land use classes that are used to define runoff coefficients
• Remote sensing data can be used to obtain almost any information that is typically obtained from maps or aerial photography.• In many regions of the world, remotely sensed data, and particularly Landsat, Thematic Mapper (TM) or Systeme Probatoire, dObservation de la Terre (SPOT) data, are the only source of good cartographic information.• Drainage basin areas and the stream network are easily obtained from good imagery, even in remote regions• Topography is a basic need for any hydrologic analysis and modeling.• Remote sensing can provide quantitative topographic information of suitable spatial resolution to be extremely valuable for model inputs. for example, stereo SPOT imagery can be used to develop a Digital Elevation Model (DEM) with 10 m horizontal resolution and vertical resolution approaching 5 m in ideal cases
• Empirical flood formulae are useful for making quick estimates of peak flow when there is very little other information available.• Generally these equations are restricted in application to the size range of the basin and the climatic/hydrologic region of the world in which they were developed.• Most of the empirical flood formulae relate peak discharge to the drainage area of the basin.• Landsat data have been used to improve empirical regression equations of various runoff characteristics
MIKE BASIN - MIKE 11s rainfall-runoff model NAM• Given rainfall and evaporation data, NAM calculates a runoff time series that is automatically assigned to MIKE BASIN for use in the river flow simulation.• NAM is a lumped, conceptual rainfall-runoff model simulating overland flow, interflow and baseflow as a function of the moisture content in each of four mutually interrelated storages:• Snow storage• Surface storage• Root zone storage
MIKE 11 is a comprehensive, one-dimensional modelling system for the simulation offlows, sediment transport and water quality in estuaries, rivers, irrigation systems andother water bodies.It is a 4th generation modelling package designed for microcomputers with DOS orUNIX operating systems and provides the user with an efficient interactive menu andgraphical support system with logical and systematic layouts and sequencing in themenus.The package was introduced in 1989 and today the number of installations world-wideexceeds 300.The hydrodynamic module of MIKE 11 is based on the complete partial differentialequations of open channel flow (Saint Venant).The equations are solved by implicit, finite difference techniques. The formulations can be applied to branched and looped networks and quasi two-dimensional flow simulations on floodplains.MIKE 11 operates on the basis of information about the river and the floodplaintopography, including man- made hydraulic structures such as embankments, weirs,gates, dredging schemes and flood retention basins.The hydrodynamic module forms the basis for morphological and water quality studiesby means of add-on modules.
MIKE21 is a comprehensive modelling system for 2-dimensional free surfaceflows applicable to studies of lakes, reservoirs, estuaries, bays, coastal areasand seas where stratification can be neglected.MIKE21 solves the vertically integrated equations of continuity and conservationof momentum in two horizontal dimensions.Like MIKE11, MIKE21 has a modular structure where water quality modules andsediment transport modules are available as add-on modules to the MIKE21hydrodynamic module.
Integrated Hydrological Modeling MIKE 3 MIKE SWMMMIKE SHE
An Integrated Hydrological ModelTraditional Models Integrated Model Evapotrans- piration Unsaturated zone Unsaturated Evapotrans- zone piration Groundwater flow Groundwater Surface Water/ Surface Water/ flow Overland flow Overland flow
MIKE SHE – An example of an integrated model Overland flowUnsaturated zone Surface waterGroundwater flow
Why not one model? MIKE 3 MIKE SWMM
Different models offer MIKE FLOODsolutions to various problems, MIKE BASIN MIKE SHE with different approach/focus/ MIKE 21 MIKE 11level of detailFlood forecasting, flood managementDam break analysisReservoir operationRiver management, navigationSediment transport, river morphologyRiver water qualityRiver ecologyGroundwater & surface water interactionWetlandsBasin-wide water resources planningSoil & groundwater contaminationWatershed managementIrrigation, canal operation
MIKE SHEApplication Areas• River Basin planning, water use/allocation• Irrigation and drainage• Wetland protection, restoration and ecology• Impacts of farming practices• Soil and water management• Effects of changes in land use• Effects of changes in climate• Contamination from waste disposal sites• Saline related problems (not released yet)
MIKE SHEFlexible Process Descriptions Processes can be mixed as required Processes run on different spatial scales Processes run on different time scales
MIKE SHEMIKE SHE has been used in hundredsof consulting and research projectsaround the world
Trends in hydrological modeling• Models → modules in integrated, flexible modeling systems• Hydrological models become integrated with other tools (GIS, statistical, economic, optimization, decision support tools, remote sensing)• Models describe natural, as well as human influences on water flow and distribution• Models describe water quality as well as quantity
MIKE 11 Hydrology Data Requirements • Detailed topography • Channel cross sections • Channel network • Control structures • Flow cond’s at boundaries
Steps in modeling1. Define purpose of modeling2. Determine model to use3. Setup model4. Calibrate model5. Apply model: • Prediction • Scenario analysis • Optimization
Model setup/input River network/Topography/Soils/Landuse Precipitation, ET Non-point Initial sources conditions Point sourcesBoundaryconditions Geology, soils
MIKE BASIN balanceswater with waterneeds availability
Setup of MIKE BASIN Diversion point Ground water Reservoir Intake Water Irrigation Runoff supply area Return flow Hydro- Catchment power Flow target Irrigation Intake area
MIKE SHE <-> MIKE BASIN MIKE SHE MIKE BASINDetailed, physically Simple, nodal basedbased routingFor process, cause- For water allocationeffect understandingFocus on soil, Focus on riverflowgroundwater processes
Constraints for modeling• Insufficient data: – Not available, non-existing – Poor quality – Not accessible• Models costly, complex, non-transparent, time-consuming• No tradition for modeling• No faith in models
Perspectives for modeling in the CPProject participants:• Think modeling from the conception of a project• Plan for data collection in coordination with modeling• Modeling as an integrated part of the project• Coordinate approaches across projectsModellers:• Provide capacity building and support to concrete projects• Continue making models more user-friendly, flexible (in complexity, scale) and integrated