Runoff and soil loss (Quantification and modeling of watershed discharge and sediment yield) Kassa Tadele (Dr.Ing) Arba Minch University
Part I. Runoff Contents 1. Fundamental Concepts 2. Generation of runoff 3. Factors Affecting runoff 4. Methods predicting watershed runoff 5. Hydrological Models Part 2: Soil loss 6. Soil erosion process 7. Impacts of soil erosion/ downstream sedimentation 8. (Revised) Universal Soil Loss Equation
Learning outcomes Students are expected Understand response mechanisms and components of watershed runoff Capable of computing runoff from watersheds using an appropriate methods Equipped with the skill of using hydrologic models to predict watershed runoff Understand erosion processes and compute soil loss from watersheds
Questions For process research, the following questions has to be clear: How can we isolate different runoff response mechanisms? What are the key state variables controlling runoff generation? What do we want to know? Volumes of storm runoff Entire storm hydrographs Deterministic prediction of peak rates of runoff from small watersheds Probabilistic prediction of peak flows (from any size of watershed) Continuous simulation of streamflow (storm and dryweather flow)
1. Fundamental concepts DRIVER area topography soils vegetation land use etc. SYSTEM REPRESENTATION RESPONSE Q
2. Formation process of surface runoff 3 generally acknowledge methods of generation of runoff Hortonian overland flow (surface runoff) Shallow subsurface runoff Saturated overland flow (return flow) The processes whereby rainfall becomes runoff continue to be difficult to quantify and conceptualize
Cont d Ground water i > f Hortonian Overland Flow PCP intensity is higher than infiltration rate, soil cannot take up rain as fast as it is falling Ground water Saturated overland flow Shallow subsurface flow Ground water water infiltrates but a layer of lower infiltration rate exists
Classification of runoff mechanism
definitions Surface runoff includes all overland flow as well as all precipitation falling directly onto stream channels. Surface runoff is the main contributor to the peak discharge. Interflow is the portion of the streamflow contributed by infiltrated water that moves laterally in the subsurface until it reaches a channel. Interflow is a slower process than surface runoff. Components of interflow are quick interflow, which contributes to direct runoff, and delayed interflow, which contributes to baseflow. Ground water flow is the flow component contributed to the channel by groundwater. This process is extremely slow as compared to surface runoff.
Cont d Thus, total streamflow hydrographs are usually conceptualized as being composed of: Direct Runoff, which is composed of contributions from surface runoff and quick interflow. Unit hydrograph analysis refers only to direct runoff. Baseflow, which is composed of contributions from delayed interflow and groundwater flow.
3. Factors affecting runoff The main factors affecting runoff: o Drainage characteristics: basin area, basin shape (form and compactness), basin slope, soil type and land use, drainage density, and drainage network topology. Most changes in land use tend to increase the amount of runoff for a given storm. o Rainfall characteristics: rainfall intensity, duration, and their spatial and temporal distribution; and storm motion, as storms moving in the general downstream direction tend to produce larger peak flows than storms moving upstream.
Watershed Factors that Affect Runoff (cont d) Size- area of watershed Topography slope of watershed Shape of watershed Aspect of watershed Geology Soil Land cover
Factors affecting Runoff (cont d) Effect of watershed area 1mm of rain on 1km 2 of watershed represents an input of 1,000 m 3 of water or about 250,000 gallons of water. If a watershed, of 10 km 2 receives an annual precipitation of 300 mm, it is inputting about 3.0 1 billion m 3.
Factors affecting Runoff (cont d) Size- area of watershed Topography slope of watershed May include drainage density effects Shape of watershed Aspect of watershed Geology Soil Land cover
Factors affecting Runoff (cont d) Topography and drainage density Slope affects stream velocity Drainage density affects travel time of precipitation to channel
Factors affecting Runoff (cont d) Size- area of watershed Topography slope of watershed May include drainage density effects Shape of watershed Aspect of watershed Geology Soil Land cover
Factors affecting Runoff (cont d) Catchments with the same area but different shapes volume of water that passes through the outlets of both the catchments is same (as areas and effective rainfall have been assumed same for both)
Factors affecting Runoff (cont d) Also need to consider the storm duration and time of concentration.
4 Method of determining Runoff Infiltration capacity curve Nonlinear loss-rate function Consider time-varying infiltration rates Index approach Uses average rate of infiltration for storm NRCS method Uses time-averaged parameters
4.1) Infiltration Index Approach Simplest procedure Objective is to divide hyetograph into direct runoff and infiltration φ index is the average rate such that the volume in the hyetograph above the index is equal to direct runoff Need hyetograph and estimate of direct runoff to determine φ index
φ index Method
4.2) NR-SCS Method
NR-SCS Method (Cont d) Q = (P I (P I a a ) 2 + S) where Q = surface runoff [L] P = precipitation [L] Ia = initial abstraction S = potential maximum soil retention [L] Note that Q represents cumulative runoff corresponding to cumulative P
NR-SCS Method (Cont d) The curve number (CN) is defined as where CN = curve number S = potential maximum soil retention Rearranging, see that S= 1000 CN 10
SCS Method -Curve Number (Cont d) Curve number related to: Hydrologic soil group Land cover, treatment and condition Antecedent moisture conditions Based on the Hydrologic Soil Group (HSG), land use and condition. Range between 0 and 100. The greater the curve number, the greater the potential for RO.
SCS Method -Curve Number (Cont d) Hydrologic Soil Groups oscs classified more than 4000 soils into four general HSG (A, B, C, and D)
SCS Method -Curve Number (Cont d) Land Use and Condition Curve numbers for various land uses ranging from cultivated land to industrial and residential districts.
SCS Method -Curve Number (Cont d)
SCS Method -Curve Number (Cont d) Antecedent Moisture Conditions Runoff potential is dependent on antecedent moisture conditions so CN is dependent on that. The CN in the table are for antecedent moisture condition II which is average soil moisture conditions CN(II). CN(I) is used when there has been very little rainfall preceeding the rainfall in question (dry soil) CN(III) is used when there has been considerable rainfall before the rainfall in question (wet soil)
SCS Method -Curve Number (Cont d)
Important! 1. Method entrenched in runoff prediction practice and is acceptable to regulatory agencies and professional bodies. 2. Attractively simple to use. 3. Method packaged in handbooks and computer programs 4. Appears to give reasonable results --- big storms yield a lot of runoff, fine-grained, wet soils, with thin vegetation covers yield more storm runoff in small watersheds than do sandy soils under forests, etc. 5. No easily available competitor that does any better. The method is already used in various larger computer models, such as SWAT, HEC-HMS).
4.3) Unit Hydrograph Method t s 1 cm direct runoff 1cm direct runoff Definition: A tr-unit Hydrograph is the DR hydrograph produced by a storm of 1 unit effective rainfall and effective rainfall duration tr. Φ Method for simulating the time distribution of a known volume of stormflow Limited to basins < 5,000 square km Unit hydrographs are specified for a known duration of effective rainfall (t s ) t b
Derivation: To derive a Unit Hydrograph (i) Separate the base flow from the streamflow hydrograph. (ii) Compute the Direct Runoff steamflow volume (area under Direct Runoff hydrograph) and divide it by the catchment s area to determine the effective rainfall depth d. (iii) Define the effective rainfall duration by separating an area equal to d from the top of the hyetograph.
Moisture Accounting rainfall-runoff models handle antecedent conditions by tracking moisture through time Applied moisture is distributed in a physically realistic manner within the various zones and energy states in soil Rational percolation characteristics are maintained Streamflow is simulated effectively
Typica Soil Moisture Accounting Model
Soil Moisture Accounting and Routing /SMAR/
4.4 Runoff Measurement Generally any hydrologic processes is measured as 1. Point Sample -Measurements made through time at a fixed location in space. -The resulting data forms a Time Series. 2. Distributed Samples -Measurement made over a line or area in space at a specific point in time. -The resulting data forms a Space Series.
Measurement Sequence Hydrologic phenomenon Sensing Recording Transmission Translation Editing Storage Retrieval Transform the intensity of the phenomenon into an observable signal Make an electronic or paper record of the signal Move the record to a central processing site Convert the record into a computerized data sequence Check the data and eliminate errors and redundant info Archive the data on a computer tape or disk Recover the data in the form required User of data
Measuring runoff The measurement of runoff may be required to assess the relative contribution of different hillslope runoff processes; i.e. throughflow, overland flow,etc. There are no standard methods for the measurement of runoff processes; Overland flow Overlandflow can be measured using collection troughs at the bottom of hillslopes or runoff plots. A runoff plot is an area of hillslope with definite upslope and side boundaries so that you can be sure all the overland flow is generated from within each plot. The upslope and side boundaries can be constructed by driving metal plates into the soil and leaving them protruding above the surface.
Cont d Several runoff plots are used to characterize overland flow on a slope as it varies considerably in time and space. This spatial and temporal variation may be overcome with the use of a rainfall simulator. Throughflow/lateralflow The only way to measure it is with lateral flow troughs dug into the soil at the appropriate height. The problem with this is that in digging, the soil profile is disturbed and consequently the flow characteristics change. It is usual to insert troughs into a soil face that has been excavated and then refill the hole.
Runoff and sediment collecting point
5) Hydrological Models Hydrological modelling is a mathematical representation of natural processes that influence primarily the energy and water balances of a watershed. simplified, conceptual representations of a part of the hydrologic cycle
Tasks of hydrological models (cont d) Modelling existing catchments for which input-output data exist, e.g. Extension of data series for flood design of water resource evaluation, operational flood forecasting, or water resource management Runoff estimation on ungauged basins Prediction of effects of catchment change e.g. Land use change, climate change Coupled hydrology and geochemistry e.g. Nutrients, Acid rain Coupled hydrology and meteorology e.g. Global Climate Models Page 43
hydrological models-components (cont d)
Classification according to process description (cont d) Empirical - Black Box (empirical) Hydrological model Conceptual Physically based Equations: No relation to theoretical process descriptions Parameters: Can not be related to field data Conceptual- Equations: Some reflection of physical concepts in theoretical process descriptions Parameters: Can some times be related to field data, but can usually not be assessed directly from field data Physically based Equations : theoretical process equations Parameters: Can in principle be assessed from field data
6) Soil erosion process Erosion is a process of detachment and transport of soil particles by erosive agents. Ellison, 1944 It is the process of moving soil (by wind or water) from place of origin to another location. EROSION IS A CONCERN Degrades soil resource Reduces soil productivity Reduces soil organic matter Removes plant nutrients Causes downstream sedimentation Produces sediment which is a pollutant Produces sediment that carries pollutants
Global land and soil degradation
Soil erosion process (cont d) 1. Detachment 2. Transport 3. Deposition Soil erosion by water is the result of rain detaching and transporting vulnerable soil, either directly by means of rain splash or indirectly by rill and gully erosion
Factors controlling soil erosion process
T value = Tolerable erosion Maximum soil erosion loss that is offset by the theoretical maximum rate of soil development, which will maintain an equilibrium between soil gains and losses. The maximum average annual soil loss that will allow continuous cropping and maintain soil productivity without requiring additional management inputs. T ranges from: 2 11 t/ha/yr (1-5 t/ac/yr) < 25 cm = 2.2 t/ha/yr > 152 cm = 11.2 t/ha/yr
7) Impacts of soil erosion/ downstream sedimentation soil erosion has a range of environmental impacts, including loss of organic matter and soil nutrients, reduction of crop productivity and degradation of downstream. there is a need to improve the understanding of erosion and deposition processes at field, catchment and larger regional scales from the quantitative perspective, in order to be able to analyse their on-site impact on soil productivity as well as their off-site impact on streams (e.g., sedimentation and water quality)
On and off-site effects of soil erosion
Rainfall VS Sediment yield
SEDIMENT CHARACTERISTICS Particle Classes Primary clay, primary silt, small aggregate, large aggregate, primary sand At Detachment Distribution of classes function of texture Diameter of small and large aggregates function of texture After Deposition Sediment enriched in fines
Deposition VS transport capacity and sediment load Transport capacity Sediment load Hillslope Transport capacity = sediment load Sediment production less Deposition than transport capacity Deposition because sediment production exceeds transport capacity
8) Erosion modeling Erosion modelling is based on an understanding of the physical laws and landscape processes such as runoff and soil formation occurring in the natural environment. Modelling translates these components into mathematical relationships, describing the fundamental water erosion processes of detachment, transport and deposition (Jetten et al., 2003).
8.1) Erosion and sediment transport models (adapted from Merrit et al. 2003)
Erosion and sediment transport models
8.2) Universal Soil Loss Equation To guide methodical decision making in conservation planning on a site basis To predict longtime average soil losses and runoff from specific areas in specified cropping and management systems. To enable planners to project limited erosion data to many locations and conditions not directly represented by research For estimating average annual soil loss from sheet and rill erosion only.
USLE- Background Combines empirical field data-process based equations (natural runoff and rainfall simulator plots) Zingg s equation (1940) Smith and Whit s equation (1947) AH-282 (1965) Undisturbed land (1975) AH-537 (1978) Disturbed forestland (1980) RUSLE1 (1992) AH703 (1997) OSM Manual (mined, reclaimed land, construction sites) (1998) RUSLE2 (2001)
USLE- EROSION PREDICTION AS A TOOL Guide management decisions Evaluate impact of erosion Inventory soil erosion Conservation planning Concept: Estimate erosion rate Evaluate by ranking Evaluate against quality criteria Tool: RUSLE2 Quality Criteria: Soil loss tolerance
USLE- PLANNING VARIABLES Soil loss on eroding portions of hillslope Detachment (sediment production) on hillslope Conservation planning soil loss for hillslope Ratio of segment soil loss to soil tolerance adjusted for segment position Sediment yield from hillslope/terraces
OVERVIEW OF RUSLE2 Where RUSLE2 applies Major factors affecting erosion RUSLE2 factors RUSLE2 background
Overland flow Interrill Rill Ephemeral Gully (Concentrated flow) Landscape RUSLE2 Area Erosion Types
RULSE A = R x K x (LS) x C x P A = Predicted Average Annual Soil Loss (tons/ac/yr) R = Rainfall Factor K = Soil Erodibility Factor LS = Slope Length and Steepness Factor C = Cover and Management Factor P = Support Practice Factor
RUSLE FACTORS (Sediment Production) Climate r Soil Topography k ls Land Use and lscp Management A = f (erodibility, erosivity) Erosivity rklscp Erodibility - klc
RUSLE2 Factors (Keep in mind that factors are on a daily basis) r- erosivity factor k- erodibility factor l- slope length factor s- slope steepness factor c- cover-management factor p- supporting practices factor
Rainfall Factor (R) Measure of rainfall energy and intensity rather than just rainfall amount. R factors values in Missouri are specific by county. Use RUSLE Rainfall-Factor Map to determine R Factor
Soil Erodibility Factor (K) Measure of the relative resistance of a soil to detachment and transport by water. Use Kf factor representing the fine-earth fraction of the soil. The fine-earth fraction represents the soil subject to sheet and rill erosion. Adjust the Kf factor using the Average Annual K Factor Table K = RUSLE Adjusted K Value
Soil Erosion (tons ha - 1 yr - 1 ) Soil Erosion class Erosion Map Index 0-5 Slight 1 5-10 Moderate 2 10-20 High 3 20-80 Very High 4 > 80 Severe 5
Slope Length and Steepness Factor (LS) Ratio of soil loss from a given field slope to that from a slope that has a length of 72.6 feet and a steepness of 9 percent. Slope Length (L) is the point water starts to flow to the point where deposition occurs or runoff is concentrated. Slope steepness (S) is the horizontal fall (ft) given in 100 feet. Use Table 1: Values for LS for Rangeland, Pastureland, Long Term No-Till Cropland Use Table 2: Values for LS for Row-Cropped Agricultural
Cover and Management Factor (C) Ratio of soil loss from land cropped under specific conditions to corresponding loss from clean-tilled, continuous fallow. One factor a landowner can impact the amount of soil loss. Figure Climatic Zone for the specific county Use corresponding C-Factor Table Figure C factor based on rotation
Support Practice Factor (P) Ratio of soil loss with a specific support practice to the corresponding soil loss with up and down the hill tillage. Support practices include contouring, stripcropping, and terracing.
USLE Equation Terms Term High Value Low Value High/Lo w Ratio R 600 50 12 K 0.49 0.02 24.5 LS 6+ 0.20 30 C 0.6 0.021 29 P 1.0 0.25 4 1/29/99 74
Uses of RUSLE2: Erosion Computations
Uses of RUSLE2: Educational Tool
Uses of RUSLE2:Management Comparisons
Uses of RUSLE2: Road Grading
Uses of RUSLE2: General Modeling
APPLICABLE PROCESSES Yes: Interrill and rill erosion Yes: Sediment yield from overland flow slope length Yes: Sediment yield from terrace channels and simple sediment control basins No: Ephemeral or permanent incised gully erosion No: Stream channel erosion No: Mass wasting
Water Erosion Prediction Project (WEPP) Developed in 1987 Physically based model that simulates the entire erosion process Can predict spatial and temporal distributions of net soil loss and deposition
WEPP Applications Hillslope A direct replacement of USLE Can predict soil loss and deposition on a slope Watershed Sediment detachment, transport, and deposition in channel systems, in addition to hillslope
GeoWEPP ESRI ArcView extension that uses DEM data to derive topography inputs
RUSLE2 vs. WEPP RUSLE2 An advanced empirical model that predicts longterm, average-annual soil erosion by water Runs on a Windows program Relies on climate, soil, vegetation, and cropping management databases Land use independent Used for erosion estimates on cropland and pastureland WEPP Process-oriented, simulated soil erosion prediction model Runs on a Windows program Relies on climate, soil, vegetation, and cropping management databases Applicable to small watersheds (640 acres) Can simulate small profiles up to large fields