A decrease in air temperature causes a decrease in the capacity of the air column for water vapor. The time elapsed from the beginning to the end of a precipitation event is the precipitation duration. In the case of the previous example, the return period corresponding to a frequency of 0.02 is 50 years.
Precipitation varies not only temporally, but also spatially, that is, the same amount of rain does not fall uniformly over the entire catchment area. In hydrological analysis, it is often necessary to determine a spatial average of the rainfall depth over the catchment. In the Thiessen polygon method, the location of the rain gauges is plotted on a scale map of the catchment and the surrounding area.
In the isohyetal method, the locations of rain gauges are plotted on a scale map of the watershed and surrounding area. The estimated value of precipitation at station X is the weighted average of the values at the four indexing stations.
HYDROLOGIC ABSTRACTIONS
Interception loss comprises two distinct elements [25] The first is interception storage ie. the depth (or volume) that remains in the foliage against the forces of wind and gravity. The second is the loss due to evaporation from the surface of the foliage, which takes place throughout the duration of the storm. In practice, however, some terms of this equation may be difficult to measure.
Under such circumstances, the neglect of the variation of infiltration rate with time is generally justified on practical grounds. In this sense, the Wmin index approximates a spatially averaged value of the final infiltration rate. The value of the constant k is estimated by assuming that for very small values of precipitation excess (Pe ≅ 0), essentially all the precipitation goes into depression storage (dVs /dPe = 1).
The fraction of the original solar radiation flux that reaches the earth's surface is called direct solar radiation. The fraction of the radiation reflected and scattered by the atmosphere that reaches the ground is called aerial radiation. Studies have shown that evaporation rates are a function of the difference between the saturation vapor pressure (expressed at the water surface temperature or, alternatively, at the overlying air temperature) and the partial vapor pressure of the overlying air.
Uncertainty about the applicability of various evaporation formulas led to indirect measurement of evaporation using evaporating pans. It allows the measurement of the integrated effect of net radiation, wind, temperature and humidity on evaporation from an open surface. Some authors consider that transpiration is independent of the available moisture in the soil, as long as it is above the point of permanent wilting, i.e. soil moisture that would cause permanent wilting.
Therefore, the reference crop evapotranspiration can be taken as the potential evapotranspiration of the reference crop (short green grass). The Blaney-Criddle formula [5, 6] is typical of temperature models for estimating evapotranspiration. The temperature efficiency index J is the sum of 12 values of the monthly heat index I (Eq. 2-43).
Priestley and Taylor [67] proposed that potential evapotranspiration be taken as the radiation part of the Penman equation (Equation 2-35 with Ea = 0) affected by an empirical constant. The Penman model is typical of the combination models (combining energy budget and mass transfer approaches) for calculating potential evaporation.
CATCHMENT PROPERTIES
The catchment division is loci of points that delimit two adjacent catchments, i.e. the collection of high points (peaks and saddles) that separate catchments that drain to different outlets. For this purpose, an equivalent circle is defined as a circle of area equal to that of the catchment area. The compactness ratio is the ratio between the circumference of the catchment and the corresponding circle.
The main watercourse (or main stream) is the central and largest watercourse of the catchment and the one that carries the runoff to the outlet. Relief ratio is the ratio of maximum catchment relief to the catchment's longest horizontal straight distance measured in a direction parallel to that of the main watercourse. The relief ratio is a measure of the intensity of the erosion processes active in the catchment area.
This refers to a dimensionless curve showing variation with elevation of the sub-area of the watershed above that elevation (Fig. 2-36). To develop this curve, the elevation of the highest or maximum point in the watershed divide, corresponding to a 0 percent area, is identified. Similarly, the subareas above each of the elevations are expressed as percentages of the total catchment area.
The mean height of the watershed is calculated from the height in percentages corresponding to 50 percent of the area. At a given point in the profile, the elevation is usually the mean value of the channel bottom. Based on Manning's equation (Section 2.4), it is assumed that the travel time of the current through each subtransistor is inversely proportional to the square root of its slope.
Likewise, the travel time throughout the channel is assumed to be inversely proportional to the square root of the equivalent slope. The catchment length (or the hydraulic length) L is the length measured along the main stream (Fig. 2-44). The main stream (or main stream) is the central and largest stream in the catchment and the one that directs the flow to the outlet.
RUNOFF
Hydrological procedures for small catchments usually require an estimate of the time of concentration (Chapter 4). The time of concentration for the entire reach is the sum of the times of concentration of the individual subreaches. A limitation of the steady flow approach to the calculation of time of concentration is the fact that the flow considered is generally unsteady.
This means that the speed of travel of the wavy features of the flow (ie, the kinematic wave speed, Chapters 4 and 9) is greater than the mean speed calculated using steady flow principles (Manning's equation). The rising leg is the part of the hydrograph between zero time and time to peak. Recession (or retreat) is the portion of the hydrograph between the time to peak and the time base.
The time base is measured from time zero to the time that determines the end of the recession. The wetted extent P is the extent of the flow area in direct contact with land. In the long term, the total amount of flow is directly related to the amount of environmental moisture, i.e. soil and air moisture.
Over the years, this has given rise to the concept of flow frequency, or more commonly, flood frequency, expressed as the average length of time (ie, return period) that it will take a given flood level to recur. in a certain place. For any given day, the ordinate of the flow-mass curve is the accumulated flow volume up to that day. The range is the difference between the maximum and minimum ordinates of the residual mass curve for a given period.
In small catchments, runoff is characterized by the event runoff coefficient C, i.e. the ratio of storm runoff depth to storm rainfall depth (Chapter 4). In the typical exorheic drainage, average annual runoff increases with ambient moisture, that is, the moisture present in soil. Determine: (a) the average rainfall intensity in the first 6 hours, (b) the average rainfall intensity for the entire duration of the storm.
During the same period, the rainfall at four index stations, each located in one of the four quadrants (Figure 2. Estimate the missing rainfall data at station Y. The annual rainfall at station Z and the average annual rainfall at 10 neighboring stations are the same follows: What is the storage tank volume required to provide a constant release rate equal to the mean of the data.
