PDF | On, Yibeltal Ayenew and others published Rainfall-Runoff Modelling First and foremost, honor is to the forgiving God who blessed and taking care level pools with a user-specified storage-discharge relationship. Praise be to God, the Cherisher and the Sustainer of the world. Without .. Figure Rainfall-Runoff Relationship for Catchment with Single Uniform. Surface. Storm analysis is an important aspect of rainfall evaluation. the rainfall-runoff relationship for any rainstorm depends on the dynamic interaction between rain.
In a high intensity storm the kinetic energy of raindrops is considerable when hitting the soil surface. This causes a breakdown of the soil aggregate as well as soil dispersion with the consequence of driving fine soil particles into the upper soil pores.
This results in clogging of the pores, formation of a thin but dense and compacted layer at the surface which highly reduces the infiltration capacity. This effect, often referred to as capping, crusting or sealing, explains why in arid and semi-arid areas where rainstorms with high intensities are frequent, considerable quantities of surface runoff are observed even when the rainfall duration is short and the rainfall depth is comparatively small.
3. Rainfall-runoff analysis
Soils with a high clay or loam content e. On coarse, sandy soils the capping effect is comparatively small.
Vegetation The amount of rain lost to interception storage on the foliage depends on the kind of vegetation and its growth stage. Values of interception are between 1 and 4 mm. A cereal crop, for example, has a smaller storage capacity than a dense grass cover. More significant is the effect the vegetation has on the infiltration capacity of the soil. A dense vegetation cover shields the soil from the raindrop impact and reduces the crusting effect as described earlier.
In addition, the root system as well as organic matter in the soil increase the soil porosity thus allowing more water to infiltrate. Vegetation also retards the surface flow particularly on gentle slopes, giving the water more time to infiltrate and to evaporate. In conclusion, an area densely covered with vegetation, yields less runoff than bare ground. Slope and catchment size Investigations on experimental runoff plots Sharma et al. In addition, it was observed that the quantity of runoff decreased with increasing slope length.
This is mainly due to lower flow velocities and subsequently a longer time of concentration defined as the time needed for a drop of water to reach the outlet of a catchment from the most remote location in the catchment.
This means that the water is exposed for a longer duration to infiltration and evaporation before it reaches the measuring point. The same applies when catchment areas of different sizes are compared. The runoff efficiency volume of runoff per unit of area increases with the decreasing size of the catchment i.
Figure 10 clearly illustrates this relationship. Runoff efficiency as a function of catchment size Ben Asher It should however be noted that the diagram in Figure 10 has been derived from investigations in the Negev desert and not be considered as generally applicable to others regions.
The purpose of this diagram is to demonstrate the general trend between runoff and catchment size. Even at the micro level there are a variety of different slopes, soil types, vegetation covers etc.
Each catchment has therefore its own runoff response and will respond differently to different rainstorm events.
The design of water harvesting schemes requires the knowledge of the quantity of runoff to be produced by rainstorms in a given catchment area. It is commonly assumed that the quantity volume of runoff is a proportion percentage of the rainfall depth.Rainfall-Runoff Relationships
Instead its value is highly variable and depends on the above described catchment-specific factors and on the rainstorm characteristics. For example, in a specific catchment area with the same initial boundary condition e. Also runoff coefficients for large watersheds should not be applied to small catchment areas. An analysis of the rainfall-runoff relationship and subsequently an assessment of relevant runoff coefficients should best be based on actual, simultaneous measurements of both rainfall and runoff in the project area.
As explained above, the runoff coefficient from an individual rainstorm is defined as runoff divided by the corresponding rainfall both expressed as depth over catchment area mm: Actual measurements should be carried out until a representative range is obtained.
Shanan and Tadmor recommend that at least 2 years should be spent to measure rainfall and runoff data before any larger construction programme starts. Such a time span would in any case be justified bearing in mind the negative demonstration effect a water harvesting project would have if the structures were seriously damaged or destroyed already during the first rainstorm because the design was based on erroneous runoff coefficients. When plotting the runoff coefficients against the relevant rainfall depths a satisfactory correlation is usually observed see Figure Rainfall-runoff relationships, Baringo, Kenya Source: Finkel A much better relationship would be obtained if in addition to rainfall depth the corresponding rainstorm intensity, the rainstorm duration and the antecedent soil moisture were also measured.
This would allow rainstorm events to be grouped according to their average intensity and their antecedent soil moisture and to plot the runoff coefficients against the relevant rainfall durations separately for different intensities see Figure Rainfall intensities can be accurately measured by means of a continuously recording autographic rain gauge.
It is also possible to time the length of individual rainstorms and to calculate the average intensities by dividing the measured rainfall depths by the corresponding duration of the storms. Runoff coefficients in relation to rainfall intensity, rainfall duration and antecedent soil moisture. Measured on loess soil with sparse vegetation. Siegert When analysing the measured data it will be noted that a certain amount of rainfall is always required before any runoff occurs.
This amount, usually referred to as threshold rainfall, represents the initial losses due to interception and depression storage as well as to meet the initially high infiltration losses. The threshold rainfall depends on the physical characteristics of the area and varies from catchment to catchment. In areas with only sparse vegetation and where the land is very regularly shaped, the threshold rainfall may be only in the range of 3 mm while in other catchments this value can easily exceed 12 mm, particularly where the prevailing soils have a high infiltration capacity.
The fact that the threshold rainfall has first to be surpassed explains why not every rainstorm produces runoff. Morin, The Institute of Earth Sciences, The Hebrew University of Jerusalem Erosion, flood control and tillage management are directly affected not only by the monthly and annual rain volume, but also by the intensity of individual rainstorms.
Storm analysis is an important aspect of rainfall evaluation. Runoff occurs whenever rain intensity exceeds the infiltration capacity of the soil, providing there are no physical obstructions to surface flow.
Rainfall storm analysis should consider therefore the sequence of intensive rain events as well as their magnitude. Long runs of information on rainfall amounts and intensities have been collected in most parts of the world.
These data provide a reliable set of information which allows one to assess water resource planning on a probability basis. Quantitative knowledge of a region's rainstorm characteristics is as important as information on soil properties. Single Rainstorm Analysis A rainstorm is defined here as a period of rain, in which the interval between rainfall segments does not exceed 24 hours.
Autographic rain recorders provide us with rain intensity and storm sequence information. A typical rain record is displayed in Figure 6. The trace on this chart is a cumulative rainfall curve, the slope of the graph being proportional to the intensity of the rainfall. Technically, rainstorm analysis begins with the manual digitization of each inflection point on the graph directly into a computer.
Computer programs are then used to carry out the required calculations. A specimen analysis of the rainstorm chart of The rainstorm was broken down into segments.
Each segment represents a time interval with uniform rainfall intensity. In Figure 7c the storm intensities are grouped regardless of their sequence. For example, segments 19 and 22 have an intensity of between mm h-1 with a total volume of 0.
In addition, the maximum rain intensity over a 30 minute period I30 was at a rate of The I30 value was chosen since it represents the rainstorm erosivity factor Wischmeier Here the intensities are higher, with a median of 22 mm h-1 and a maximum intensity over 30 minutes of 38 mm h Soil Infiltration On many cultivated or bare fields the soil infiltration ability is commonly limited by surface crusting rather than by deeper profile conditions.
As a result, rainwater falling on bare ground cannot penetrate, and runs off sideways, even on very gentle slopes.