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Table 2.1 Hydrological soil group classifications
Group Soil Characteristics
Minimum Infiltration Rate inh
A Deep sand, deep loess, and aggregate silts
0.3 - 0.45 B
Shallow loess and sandy loam 0.15 - 0.30
C Clay loams, shallow sandy loam, soils in organic
content, and soil usually high in clay 0.05 - 0.15
D Soil that swell upon wetting, heavy plastic clay, and
certain saline soils 0 - 0.05
2.3.2 Unit Hydrograph of Watershed
Unit hydrograph of watershed is the direct runoff hydrograph resulting from one unit of effective rainfall occurring uniformly over the watershed at a
uniform rate during a unit period of time Singh, V.J. 1992. Actually, the unit hydrograph is representing the effect of rainfall in particular basin. It is a
hypothetical unit response of the watershed to a unit input of rainfall. The unit hydrograph firstly developed by Sherman in 1932. Unit hydrograph will use to
determining the surface or direct runoff hydrograph from the effective rainfall hyetograph ERH.
The fundamental assumptions implicit in the use of unit hydrographs for modeling hydrologic systems are:
1. Watersheds respond as linear systems. On the one hand, this implies that the proportionality principle applies so that effective rainfall intensities volumes
of different magnitude produce watershed responses that are scaled accordingly. On the other hand, it implies that the superposition principle
applies so that responses of several different storms can be superimposed to obtain the composite response of the catchment.
2. The effective rainfall intensity is uniformly distributed over the entire river basin.
3. The rainfall excess is of constant intensity throughout the rainfall duration. 4. The duration of the direct runoff hydrograph, that is, it’s time base, is
independent of the effective rainfall intensity and depends only on the effective rainfall duration.
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Hydrologic system is said to be a linear system if the relationship between storage, inflow, and outflow is such that it leads to a linear differential equation.
The hydrologic response of such systems can be expressed in terms of an impulse response function through a so-called Convolution Equation. Linear systems
possess the properties of additively and proportionality, which are implicit in the convolution equation Ramírez, 2000.
The impulse response function of a linear system represents the response of the system to an instantaneous impulse of unit volume applied at the origin in
time t=0. The response of continuous linear systems can be expressed, in the time domain, in terms of the impulse response function via the convolution
integral as follows,
= − ,
ℎ = 0 0 … … … … … … … … … … . 5
Where ut represents the instantaneous unit hydrograph, and Qt and Iet represent direct runoff and excess or effective precipitation, respectively.
