Wherever in the world, a portion of the water that falls as rain and snow, infiltrates into the subsurface soil and rock but extent of this infiltration vary locally. Infiltration depends on a number of factors such as intensity of rainfall, type of soil, infiltration capacity and evaporation capacity.
A portion of water that infiltrates will remain in the shallow soil layer where it will gradually move vertically and horizontally through the soil and subsurface material. Another part of the water may infiltrate deeper, recharging groundwater aquifers. If the aquifers are porous enough to allow water to move freely through it, people can drill wells into the aquifer and use the water for their purposes. Water may travel long distances or remain in groundwater aquifers for long periods before returning to the surface or seeping into other water bodies, such as streams and the oceans.
When rainfall exceeds the infiltration rate at the surface, excess water begins to accumulate as surface storage in small depressions. As depression storage fills up to its maximum, overland flow or sheet flow may begin to occur and this flow is called as “Surface runoff”
- Amount of rainfall: The runoff has a positive correlation with the rainfall. i.e. as the rainfall increases, the chance of increase in runoff will also increases.
- Soil type: Infiltration rate depends mainly on the soil type. If the soil has more void spaces, then the infiltration rate will be higher which ultimately more reduce surface runoff.
- Evaporation capacity: If the evaporation capacity is higher, surface runoff will be reduced.
Overland Flow or Surface Runoff
After a particular storm a portion of water which travels over the surface of the ground is known as the overland flow or surface runoff. Surface runoff extremely depends on permeability of the ground surface. It has much higher values over impermeable urban ground areas than over permeable soil layers. Therefore, surface runoff has its limitation since it may only occur over a permeable soil surfaces when the rainfall rate exceeds the local infiltration capacity.
Interflow or Subsurface Storm Flow
As a result of a precipitation, a portion of water that infiltrated into soil ultimately starts to move laterally through the upper soil layers until it reaches a stream channel. This subsurface water movement is called as interflow
Groundwater Flow or Base Flow
The portion of precipitation that percolates downward until it reaches the water table is known as the base flow or groundwater flow. This water accretion may eventually discharge into the streams if the water table intersects the stream channels of the basin. However, its contribution to stream flow cannot fluctuate rapidly because of its very low flow velocity.
Hydrograph is a plot which shows the variation of discharge with time. It has three regions called as rising limb, crest segment and falling limb. Nature of hydrograph depends on rainfall and watershed characteristics.
Figure 1. Typical Hydrograph (shows variation of discharge with Time)
- Rising limb: Ascending portion which represents the rising of discharge with gradual increasing of in-flow into stream. Slope depends on storm and basin characteristics.
- Crest Segment: Inflection point on rising limb to falling limb. It indicates the peak flow and is controlled by storm and watershed characteristics.
- Falling limb (recession limb): It starts from point of inflection at the end of crest segment and extend up to base flow. Inflection point indicate the time at which rainfall stopped. Slope is independent on storm characteristics but dependent on watershed characteristics.
- Climatic factors: Form of precipitation, Duration of rainfall, Distribution of rainfall, Direction of storm movement.
- Physiographic factors: Shape of basin, Size of basin, Stream slope, Drainage density, Landuse, Surface depression.
To determine relationship between rainfall and runoff, the hydrological apparatus can be used. This apparatus enables many hydrology phenomena to be studied in the laboratory. The apparatus consists of a large stainless steel tank to be filled with graded sand to form the catchment and experimental area. The sand bed can be elevated up to 1:40 by worm jacks. Rainfall is provided by two independently controlled sets of four special spray nozzles.
At each end of the catchment area are end compartments, separated from the catchment by weir plates with porous ‘port holes’. A pump takes water from the reservoir and feeds it to the overhead nozzles and to the ends of the catchment area. Calibrated rectangular weir under the catchment area can be used to measure flow from the wells or the tank.
Figure 2. Hydrology apparatus
There are several experiments which can be done using this apparatus. They are,
- Investigation of rainfall/run-off relationships for dry, saturated and impermeable catchments of various slopes (surface run-off only)
- Effect of interflow on outflow hydrograph surface runoff (plus groundwater flow)
- Simulation of multiple and moving storms
- To understand the rainfall-runoff relationship of a catchment.
- To understand how to use the hydrology apparatus.
- To understand the effect of permeability of the ground surface for infiltration and runoff.
- Nine rain gauges were randomly placed on the model catchment.
- Valves were opened for the sprinklers to form an artificial rain over the model catchment.
- Height of the water level in the weir was measured at each fifteen seconds for a time period of ten minutes.
- After ten minutes, rain was allowed stop and the water level in the weir was further measured until it reaches the zero level.
- Location of each rain gauge on the catchment was measured.
- Also, the rainfall volumes in each rain gauge were measured.
- This procedure was repeated after covering the model catchment using a polythene sheet simulating an urbanized catchment.
Mean precipitation (ẋ) =
A1= Area of the influential polygon
P1= Rainfall recorded by the rain gauge
A = Total area
Mean precipitation (ẋ) = ∑X / n
n= no. of rain gauges existing
∑X = total precipitation
Standard deviation (σ) =
Coefficient of Variation (Cv) =
Appropriate no. of rain gauges (N) =
E= Allowable percentage error
Rainfall intensity = Total height of rain gauges
Direct flow = Discharge – Base flow
According to Thiessen’s polygon method,
Table 1. Table of station, volume of precipitation, precipitation, area of influential polygon and the product of area and precipitation
|Station||Volume(mL)||Precipitation(P) (cm)||Area(A1)(km2)||A1* P1(km2cm)|
|∑ = 2.05||∑ = 5.2693|
Mean precipitation =
= 5.27 km2cm / 2.05 km2
= 2.57 cm
Table 2. Table of station, precipitation, mean precipitation, (precipitation –mean precipitation) and (precipitation –mean precipitation)2
|Station||Xi(cm)||ẋ(cm)||XI – ẋ(cm)||(XI – ẋ)2(cm2)|
|∑ = 17.42||3.962|
ẋ = ∑X / n
= 17.42cm / 6 = 2.90 cm
= 100 * 0.8902/ 2.90 = 30.6966
Appropriate number of rain gauges = 2
If allowable error (E) is 10%,
= 942.2813 / 100 = 9.42
Table 3. Water height of the weir at each 15 seconds
|Time(s)||Height(mm)||Discharge (m3s1)(10-4)||Base flow|
Figure 3. Graph of Flow rate Vs Time
Rainfall intensity = Total height of rain gauges
= 17.42cm/ 10 min= 1.742 cm min-1
Table 4. Water height at the weir after covering the catchment with a polythene
Figure 4. Graph of Flow rate Vs Time
Table 5. Table of station, rainfall volume and precipitation
Rainfall intensity = Total height of rain gauges
= 17.33 cm/ 10 min = 1.733 cm min-1
The accuracy of this experiment can be increased by repeating it at least three times for both situations.
Before starting the experiment, it is better to calculate the appropriate number of rain gauges needed to cover the catchment area with that specific rainfall intensity. Then inflow values may be much more accurate.
Presence of some water content in the apparatus at the start of the experiment could cause for errors in results. When placing the rain gauges, their locations must be fair enough to give correct rainfall values. They should not be located directly under the sprinklers and should be evenly distributed over the model catchment area.
Unusual rainfall volumes in rain gauges D, G and I was observed during the experiment. Therefore, these values were neglected when calculating the mean precipitation and number of appropriate rain gauges. These stations may have obtained such extreme values as they were not evenly placed on the catchment.
Even we have done above steps to avoid the errors, there are many other errors which may result in unexpected results,
- Errors in recording the water height
- Errors in measuring the rainfall volumes
- Errors in measuring the area of polygons
By minimizing above errors, expected patterns and relationships can be taken using this hydrological apparatus.
By changing the media of the apparatus, we can determine how the infiltration and runoff patterns change with different soil types. Since the slope of the model catchment can be changed, effect of the slope to the runoff can be experimented using this apparatus.
Hence, this apparatus is very useful to determine the relationship between rainfall and runoff in different conditions.
According to the results in both conditions, discharge has increased at first and then it has declined. In the plotted hydrograph we can clearly observe this pattern. Hydrograph only represents the surface runoff produced by the direct accumulation of water. But there is an another important component which called as the base flow. So base flow has been separated from the hydrograph and it clearly shows that the base flow is also increasing with the time. With a specific storm action, base flow increases and contributes to the runoff.
Mean precipitation over the catchment is 2.57 cm according to the Thiessen’s polygon method. But the arithmetic average depth of rainfall is 2.90 cm. However, the value obtained from the Thiessen’s polygon method is more accurate.
With the allowable error of 10%, the appropriate number of rain gauges is nearly 10. In order to decrease the error, number of rain gauges should be increased.
Impervious cover is a surface in the landscape that cannot effectively absorb or infiltrate rainfall. When natural landscapes are intact, rainfall is absorbed into the soil and vegetation. These mediums naturally slow down, spread out, and soak up precipitation and runoff. Water percolating into the soil becomes a stable supply of groundwater, and the runoff is naturally filtered of impurities before it reaches creeks, streams, rivers, and bays.
In this experiment the catchment was covered with a polythene to simulate an impervious cover. Results indicated an increase in direct runoff after covering with this polythene.
As areas become more developed, the amount of impervious coverage also increases. So natural filter systems cannot intercept the runoff anymore. This has serious implications for water quality and flood control. There are several pollutants in the runoff coming from impervious areas. They include pesticides, oil, litter, fertilizers, sediment, salt, and bacteria. Groundwater recharge, stream base flow, and water quality measurably change and can decrease as impervious cover increases.
Actually it seems that there is a direct relationship between the impervious covers and the runoff. With the increasing of impervious cover on land surface, water volume that infiltrated to recharge base flows also reduces. Therefore, this results increases in runoff after storms.
As more development and urbanization carry out, more of the natural landscape is replaced by impervious surfaces, such as roads, houses, parking lots, and buildings. This reduces the infiltration of water into the ground and accelerates the runoff. In addition to increasing imperviousness of land surface, other human actions such as removal of vegetation and soil, grading the land surface, and constructing drainage networks also contribute to increase the runoff volumes and shorten the runoff time into streams from rainfall and snowmelt. As a results of this, the peak discharge, volume, and frequency of floods increase in nearby streams.
Actually urbanization can have a great effect on hydrologic processes such as surface-runoff patterns. Therefore Local governments should encourage, assist, or require builders to minimize impervious surfaces.