10 Runoff estimates from watersheds and hydrological modeling

Objectives

To understand runoff Methods of Estimating runoff Application in hydrological modeling

Keywords

Runoff, overland flow, hydrologic condition, rational method, CN method, hydrograph

Introduction

The simple meaning of runoff is the flow of precipitated surface water towards river, pond and ocean. In another word, runoff is a movement of water to a channelised stream, after it has reached the ground as precipitation. The movement can occur either on or below the surface and at differing velocities (Davie, 2008). The surface runoff is a portion of water that flow over the surface and reaches the river, pond and ocean. During precipitation, initially, rain water to immerse into the ground until the soil becomes saturated. After the soil saturated, the excess water on the surface to flow into nearby low-lying area in the watershed. This excess water that flowing over the land is known as surface runoff (figure 1).The estimation runoff is required for the management of soil and water resources by suggesting the conservation structures such as flood forecasting, water balance studies, environmental impact assessment and engineering planning like dams, spillways and outlets or any waterways.

Figure 1: Hydrological cycle (Source: https://water.usgs.gov/edu/watercycle.html)

Generally, Runoff occurs only when the rate of precipitation, i.e. intensity, exceeds the rate of infiltration. Once the infiltration rate is exceeded, water begins to fill the surface depressions which is called surface or depression storage. After the depression are filled water flowing over the land as overland flow. Runoff is that portion of rainfall which moves down to the stream, channel, river or ocean as surface or sub-surface flow. Overland flow is the water which runs across the surface of the land before reaching the stream. In the subsurface, through flow (some authors refer to this as lateral flow) occurs in the shallow subsurface, predominantly, although not always, in the unsaturated zone. Groundwater flow is in the deeper saturated zone. All of these are runoff mechanisms that contribute to stream flow.

The run-off modeling is very much applied in the decision making in the hydrological system and their problems such as flood, contamination studies, etc. rainfall-run-off modeling can be carried out within a purely analytical framework based on observations of the inputs and outputs to a catchment area. the catchment is treated as a ‘black box’ without any reference to the internal processes that control the rainfall to runoff transformation. Information on run-off and sediment yield could provide light on the degradation of watersheds and thus identifies critical source areas for starting development programs.

Factors affecting the runoff

Physiography: It includes the shape and size of the watershed, slope, drainage pattern and density. The shape of the watershed determines the time of concentration. Long and narrow watershed have longer time of concentration resulting lower runoff than circular or square watershed because it takes long time to leave watershed and more opportunity for water to soak into the soil. The size of the watershed determines the runoff volume and rate. As the watershed size increase, runoff volume and rate increases. However, runoff volume and rate per unit of watershed area decrease as the area increases. So, watershed size and area determines the peak rate of runoff, which is important parameter for designing erosion structures. The slope of the watershed determines the velocity and extent of the runoff water. There is positive correlation between land slope and velocity, as greater the slope, greater the velocity. According to the Galileo’s law of falling bodies, distance traveled by a falling body is directly proportional to the square of the time it takes to fall. Hence, if the watershed slope is increased four times, the velocity of water flowing on the slope is doubled, resulting cutting capacity or erosive power is increased four times as well as carrying capacity of water increased about 32 times and size of particles that can be transported by rolling or pushing is increased by 64 times. The drainage pattern is a design of stream courses and their tributaries which is influenced by structure, lithology and slope. Dendritic pattern has fine drainage texture indicates the impervious rock formation and low permeability. Here, the soil is deep, heavy and slowly permeable, prone to severe to very severe soil erosion forming gullies due to high runoff. The radial, braided and pinnate drainage pattern associated with the medium drainage texture, the rocks are characterized by joints and fractures, soil is moderately deep, medium soil texture and moderately permeable subject to moderate to severe soil erosion. The coarse drainage texture of the trellis, rectangular and annular drainage pattern generally associated with shallow and coarse soil texture resulting high hydraulic conductivity where water can move through pours and fractures very easily, is relatively less prone to soil erosion. The drainage density also influencing the runoff pattern. Higher the drainage density increases the runoff water very rapidly, decreases the lag-time and increases the peak of hydrograph.

Soil and geology: The soil and geological characteristic determine the amount of siltation in the water harvesting structures such as dams and reservoirs as well as amount of water percolation in the soil.

 Precipitation: the amount and nature of precipitation is the most important factor which determines the runoff in a watershed.

Quantity and rate of runoff: The quantity and rate of runoff is prerequisite for the planning and designing of canals, ditches or any other water harvesting structures to store the runoff. The volumetric assessment of water is most important while impounding or creating reservoirs. The rate of runoff is essential for conveying water from one place to another place by creating mechanical structures to accommodate the flow of water. Theoretically, watershed with impervious rocks and no loss of water, maximum rate of runoff would be directly proportional to the rate of rainfall. So, more the rainfall, more runoff. But in the natural condition, there are various interceptors such as vegetation, infiltration into the soil, stored in depression, and some rainfall is evaporated. Therefore, estimation of rate of surface runoff is depends on the rate of rainfall and water available for runoff.

Time of concentration: when the rainfall occurs, the water take time to reach the outlet. Thus, time taken for water to travel by overland flow from any point of watershed to the outlet is known as time of concentration. There are various factors that effecting the concentration time such as size, shape and slope. The larger the watershed will take more time to concentration of water, and vice versa. The shape of the watershed is determined by shape factor, which is defined as the ratio of basin area, to the square of basin length (Horton, 1945).The value of form factor would always be less than 0.7854 (for a perfectly circular basin). the smaller the value of form factor, more elongated will be the basin. The basins with high form factors have high peak flows of shorter duration, whereas, elongated sub-watershed with low form factors have lower peak flows of longer duration. If the basin size is same, flood flows of such elongated basins are easier to manage, than those of the circular basin. The slope of the drainage basin increases the runoff as well as time of concentration is also very less.

Intensity, duration and frequency of rainfall: The relationship between these three components of rainfall is very important while planning for various rainwater harvesting structures. Theintensity and duration of rainfall has an inverse relationship, higher the intensity of storm, lower the duration of rainfall and vice versa. The most severe rainfall lasts for shorter time and storm that last for longer duration give the more total amount of rainfall, that gives impact on runoff.

Hydrologic condition of soil: the hydrologic condition of the soil is determined by the moisture content at the time of the storm. Soil with humus, organic content and temperature increase the capacity of moisture content, resulting volume of runoff and vice versa.

Vegetation cover: The canopy cover of the vegetation act as interceptors and create barriers along the flow of water, reduce peak discharge, so the type and quality of vegetation on land influence the runoff, infiltration rates, erosion and sediment production and the rate of evaporation. The foliage and its litter maintain the impact of rainfall and increase the potential of infiltration to the soil and also protect soil erosion. Thus, the canopy cover and ground litter create barriers to the water to flow on the land surface resulting increased time of concentration and it reduces the peak discharge rate.

Land use/cover and conservation practices: The land in the drainage basin consist of various natural cover and different uses by the human being such as forest, agriculture, built-up area, etc. affects the rate of runoff and infiltration. Forest and agriculture act as interceptor and increases the infiltration but built-up area accelerates runoff and reduce the infiltration of rainwater to ground. The conservation practices reduces the volume of runoff as well as soil erosion. Contouring and terracing conservation methods reduces the sheet erosion of soil and increases the amount of rainfall withheld from by the small reservoirs.

METHODS OF RUN-OFF ESTIMATION

There are various methods for runoff estimation which is suitable for the small watershed such as rational method, Curve Number Method, cook’s method, table method and hydrograph. For applying these method, two assumptions are made; one is, rainfall occurs with uniform intensity over the entire drainage basin and secondly, rainfall occurs at uniform intensity for duration at least equal to the time of concentration of the drainage basin.

Rational method

This method is widely used for the estimation of peak rate of discharge from the small watershed, due to its simplicity and easy application, developed by Kuichling (1889) for small drainage basin in urban areas. This method relates runoff producing potential of the watershed, the average intensity of rainfall for a particular length if time, and the watershed drainage area. It is expressed by the equation:

Where, ‘Q’ is peak rate of runoff in cum/sec for the given frequency of rainfall; ‘C’ is rational runoff coefficient having values ranging from zero to one depending upon watershed condition; ‘I’ is intensity in mm per hour for design frequency and for duration equal to time of concentration; and, A is an area of watershed in hectares.

The runoff coefficient ‘C’ is a dimensionless ratio intended to indicate the amount of runoff generated by a watershed given an average intensity of rainfall for a storm. Value of ‘C’ for different slopes and land use conditions are determined from field observations (table 1). When catchment area with different value of ‘C’, the weighted value of ‘C’ should be calculated for the whole watershed. The storm intensity is a function of geographic location and design exceedance frequency. It is true that longer the return interval, greater the rainfall intensity for a given storm duration. Also, longer the length of the storm, lower the storm average rainfall intensity.

Table 1: General runoff coefficient used in rational method

Though rational method is simple but it also has some limitations. There is hardly ever a rainfall completely satisfying both the assumptions of uniform intensity for at least the duration of the time of concentration and over the entire area of the watershed. This method is applied to very small watershed with an area less than 1300 hectare. The value of runoff coefficients is based on studies on a broad range of topography, soil and land use condition. But small watersheds are mostly affected by land use, tillage and cropping pattern as well as the effect of different conservation and agronomical practices, that should be considered for estimation of coefficient. In spite of limitations, this method is considered accurate for estimating runoff for relatively inexpensive water harvesting structures.

Cook’s method

The Cook’s method is developed by an engineer of the United State Soil Conservation Service. This method is considered most suitable for watershed up to 400 hectares. The runoff characteristic of a watershed is examined under the four categories of relief, soil infiltration, vegetation cover and surface storage. The approximate weightage is obtained from table 2. The peak runoff is obtained by the runoff curve and drainage area for 10 years. The peak runoff value is modified by multiplying with the rainfall, frequency and shape factors.

Table 2: Runoff producing characteristics from watershed

   The curve number method

This method is based on the recharge capacity of the watershed which is determined by antecedent moisture conditions and by the physical characteristics of the watershed. This method is most rational and realistic method determines both peak rate and volume of runoff for small watersheds by synthesizing information about the soil cover, physiographic features and flow characteristics. This method is also known as Hydrologic Soil Cover Complex Number Method. A combination of specific soil and specific cover is referred to as soil cover complex and a measure of this complex is used in this method in estimating peak rate.It is calculated by following formula:

Where Q= accumulated storm runoff in mm; P = accumulated storm rainfall in mm; S = Potential maximum retention of water by the soil.

To simplify the above equation empirical relationship between the variables S and Ia was developed from the data collected from various watershed in U.S.A. resulting the following equations:

For AMC I , Ia = 0.3S

For AMC II, Ia = 0.2S

For AMC III, Ia = 0.1S

Where     = 25400 − 254

For Indian condition Central Soil and Water Conservation Research & Training Institute, Dehradun has suggested

Ia = 0.1S (for black soil region AMC II & III)

Ia = 0.3S (for soil region AMC I)

Ia = 0.3S (for all other regions.

There are three basic data is required for this method

1. Antecedent moisture condition (AMC) which is the index of soil condition with respect to runoff potential before storm and it has three categories (table 3)

Table 3: Rainfall limit for estimating Antecedent Moisture Condition (AMC)

2. Hydrologic Soil Group (HSG), it can be categories into four groups on the basis of intake of water on bare soil when thoroughly wetted. (table 4)

Table 4:Hydrologic Soil Groups

3. Land use /cover data with their hydrologic condition (table 5)

Table 5: CN for Hydrologic Soil Group for watershed condition II (AMC II)

For example: Given a 8.122 ha farm with an average slope 0.9 per cent. The watershed is under straight row crops and falls under the hydrologic soil group B. two-year rainfall of 24 hr duration is 95.25 mm. compute the runoff depth in mm.

Solution is CN for given land use and hydrologic soil group B is 86 (table 5)

Table method

There are wide variations in the annual rainfall amount from low rainfall to severe storms and the difference in runoff are mostly influenced by variation in the surface conditions than the variations in the rainfall intensity. A single set of tables can be used for determining the peak rate of runoff throughout the country. The high rate of runoff is influenced by the shape of the watershed such as square, long and narrow so the separate table are used for determining the runoff from the watershed upto 500 acres (table 6,7 and 8). The field investigation of the watershed for the determination of its characteristics such as vegetation cover, slope and soil type (table 9).

For example: A 30 acres square watershed of bare soil (25) of fair permeable and depth (25) with a moderate slope (10), has a sum of watershed characteristics of 60 (25+25+10 = 60). The peak rate of runoff os 76 (cfs) cubic feet per second.

Table 6: Runoff from a square watershed in (cfs)

Table 7: Runoff from a broad and short watershed in (cfs)

C = total Peak discharge from watershed characteristics

Table 8: Runoff from a long and narrow watershed in (cfs)

Table 9: Peak discharge from watershed Characteristics

Hydrograph

Hydrograph provides the rate of flow or peak flowat all points in time during and after storm or snowmelt event. Because a hydrograph plots volumetric flow rates against time, integration of the area beneath a hydrograph between any two points in time gives the total volume of water passing the point of interest during the time interval. The hydrograph in figure 2 show two graph, one is rainfall, measured in mm and another is water discharge measuredin cubic metres per second (cumecs)from watershed. The highest rainfall in a graph shown by peak rainfall and highest discharge by peak discharge. There are two limbs in discharge graph one is rising limb and another is falling limb. The time between highest rainfall and highest discharge is known as lag time. Thus, in addition to peak flows, hydrographs allow analysis of sizes of reservoirs, storage tanks, detection of ponds and other facilities that deal with volume of runoff.

   Application of runoff estimation in hydrological modeling

   Beside other factors, runoff estimation is most important for the planning of reservoirs. The capacity of the irrigation canals, installed capacity of power houses will depends on the available supplies of water. The hydrological investigation are required to give detail information about the runoff pattern at the proposed project site, to determine the storage capacity of reservoirs. Determination of hydrograph of the flood is important to determine the spillway capacity and design.

Runoff potential map is important for the construction of rainwater harvesting structure such as farm pond, percolation tank and ground water recharge. The Thornthwaite’s and Mather’s model can be used for calculating the surface runoff potential zones by using rainfall, temperature, soil, land use and vegetation.

Hydrological modelling can be used for watershed conservation planning. Watershed is an area from which runoff, resulting from precipitation, flow past a single point into large river, lake or ocean. Each watershed is an independent hydrological unit and any modification reflects on the runoff pattern and sediment yield of watershed. The protection of watershed implies the proper use of all land and water resources for optimum production and minimum hazard to natural resources. The main objective is to manage the watersheds are to control damaging runoff such as flood and utilize for useful purpose such as enhance the ground water storage.

References

• Andrews, R.G., (1954). The use of relative infiltration indices in computing runoff”. Rainfall-runoff relationship, V. P. Singh, ed., Water ResourcesPublications, Littleton, Colo,
• Beven, K. (2012). Rainfall-Runoff Modelling: The Primer, Second Edition, John Wiley & Sons, Ltd, Chichester, UK
• Chow, V.T. (1986). Handbook of Applied Hydrology. McGraw-Hill, New York.
• Das, G. (2012). Hydrology and Soil Conservation Engineering, PHI Learning Private Limited, New Delhi, pp.80-83
• Hawkins, R.H. (1983). Discussion of Antecedent moisture condition probabilities. Journal of Irrigation and Drainage Engineering 109: pp.298-299,
• Mishra, S.K., Singh, V.P., (1999). Another Look At SCS-CN Method. Journal of Hydrologic Engineering, 4: pp. 257-264.
• Mutreja, K.N (1990). Applied Hydrology. Tata McGraw Hill Publishing Company Ltd., New Delhi
• Soulis, K.X., Valiantzas, J.D., Dercas, N. and Londra, P.A. (2009). Investigation of the direct runoff generation mechanism for theanalysis of the SCS-CN method applicability to a partial areaexperimental watershed. Hydrology and Earth System Sciences, 13, 605-615
• Tideman, E.M. ( ). Watershed Management: Guidelines for Indian Conditions, Omega Scientific Publishers, New Delhi.

Internet sources:

• https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/water/manage/hydrology/?cid= stelprdb1043063