LITERATURE REVIEW
2.2. HYDROLOGICAL MODELS AND THEIR APPLICATION TO LANDUSE PLANNING
2.2.1. RAINFALL RUNOFF MODELS
The most well known models for rainfall runoff modeling are the Rational method, the Unit hydrograph method and the SCS-CN method.
Rational method for estimating the peak discharge from small urban and rural watershed was introduced in US by Emil Kuichling in 1889 (Viessman and Lewis, 2008). This method is traditionally used to size storm sewer, channel and other drainage structures. Rational method assumes that rainfall duration equals to the time of concentration those results in the greatest peak runoff.
The rational method has wide applicability due to its simplicity (Zoppou 2001;
Hayes and Young, 2006). Rational method assumes a factor to consider all the losses while calculating runoff (infiltration, evaporation, transpiration, initial abstraction and depression storage). This factor is called runoff coefficient (C) and the value of C is determined through observation and experience.
The rational formula is:
3600000
Q= CiA (2.1)
Where, Q = Peak rate of runoff (Cumec)
C = Runoff coefficient
i = Maximum intensity of runoff for the time of concentration of the selected design storm. (mm/hr)
A = Area (m2)
The major disadvantage of the Rational method is that it provides only a peak discharge from a watershed, not a complete hydrograph, and therefore it cannot be used for routing multiple flows toward a single outlet. However, for design of drainage system, most important information required is the peak discharge. So the rational method remains the simplest way to obtain estimates of peak discharge and average runoff rates for the design of drainage system.
The concept of Unit Hydrograph was first introduced by Sherman in 1932 (Viessman and Lewis, 2008). A unit hydrograph is the hydrograph of direct runoff (excluding the base flow) for any storm that produces exactly 1 unit of net rainfall (the total runoff after abstractions).
The unit hydrograph method has the advantage of providing a complete storm hydrograph for describing the rainfall runoff relationship (Cavallini, 1993). While dealing with complex watersheds, the storm hydrograph for each sub watershed can be computed independently by the unit hydrograph method for subsequent routing down the main channel. However, the chief weakness of this method is involving the determination of infiltration loss.
The unit hydrograph method is more suitable than rational method for large watersheds. Various other methods that are based on the concept of Unit Hydrograph are also developed by different investigators to study rainfall runoff behavior. Agirre et al.
(2005) proposed the Geomorphological Unit Hydrograph of Reservoirs (GUHR) and compared the model with Nash’s Instantaneous Unit Hydrograph (Nash’s IUH) while applying it to Aixola watershed of Northern Spain. Both the models showed similar behavior, however GUHR was more preferred for considering the watershed morphology.
The SCS-CN model for small watersheds has been used extensively. The SCS-CN (orcurve number method) was developed by the USDA Natural Resources Conservation Service (SCS, 1956). This is an empirical method and widely for determining the approximate amount of direct runoff from a rainfall event in a particular area. The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic condition.
The popular form of SCS-CN method is
S Ia P
) Ia P Q (
2
+
−
= − (2.2)
where
Q is runoff ([L]; in) P is rainfall ([L]; in)
S is the potential maximum soil moisture retention after runoff begins ([L]; in)
Ia is the initial abstraction ([L]; in), or the amount of water before runoff, such as infiltration, or rainfall interception by vegetation; and it is generally assumed that Ia = 0.2S
The runoff curve number method, S and CN, is related as 10
CN 1000
S= − (2.3)
CNhas a range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. The lower curve number indicates more permeable soil condition.
Mack (1995) developed an interactive computer model called HER (Hydrological Evaluation of Runoff) with the SCS-CN method. Patil et al. (2008), while indicating the natural resources conservation services curve number (NRCS-CN) method as one of the most widely used methods for quick and accurate estimation of surface runoff from ungauged watersheds, developed an ArcGIS interface to estimate the surface runoff by adopting the NRCS-CN technique and its three modifications. The developed interface was validated using the recorded data for the periods from 1993 to 2001 of a gauged watershed, Banha, in the Upper Damodar Valley, Jharkhand, India and concluded that the application of the modified CN I method in the ungauged watersheds that are hydrologically similar to the Banha watershed would result in an accurate surface runoff
estimation. Shi et al. (2009a) determined the initial abstraction ratio (Ia) in an experimental watershed in the Three Gorges Area of China, by analyzing measured rainfall-runoff events and compared the performance of the traditional and modified ratio of initial abstraction (Ia) to maximum potential retention (S) with observed rainfall-runoff data. This Ia/S-adjusted SCS-CN method appears to be better for runoff prediction in the Three Gorges Area of China.
However, the curve number does not provide the flexibility that the rational method C value gives in evaluating site characteristics, since the SCS model was designed for rural and agricultural areas. In addition, the SCS-CN model cannot use intensity-duration-frequency data of precipitation, which, if available, improves the accuracy of the rational methods.