36 Hydrometeorology

P. P. Adhikary and Jyotiprava Dash

1. Learning outcomes
2. Introduction
3. Hydrometeors
4. Hydrological cycle
5. Rainfall
5.1. Rainfall analysis and characterization
5.1.1. Mass curve
5.1.2. Hyetograph
5.1.3. Missing rainfall data estimation
5.2. Normal precipitation
5.3. Frequency of point rainfall
5.4. Spatial measurement of rainfall
5.4.1. Arithmetic mean method
5.4.2. Thiessen polygon method
5.4.3. Isohyetal method
6. Evapotranspiration: methods of measurement
6.1. Evaporation
6.1.1. Measurement of evaporation
6.2. Transpiration
6.3. Evapotranspiration
6.3.1. Measurement of evapotranspiration
6.4. Estimation of evapotranspiration
6.4.1. FAO Penman-Monteith method
7. Measurement of runoff and stream flow
8. Summary

1. Learning outcomes

After reading through this module, one should be able to:

know what is hydrometeorology and its general importance understand different types of hydrometeors distinguish between different process involved in hydrological cycles analyze, characterize distribution of rainfall in space and time know different methods of evapotranspiration measurement know the common methods of measurement of runoff and stream flow

2. Introduction

Hydrometeorology is a branch of meteorology and hydrology that studies the transfer of water and energy between the land surface and the lower atmosphere. It is the scientific study of the interaction between meteorological and hydrological phenomena, including the occurrence, motion and changes of the state of atmospheric water and the land surface and sub-surface phases of the hydrologic cycle.

3. Hydrometeors

Hydrometeors are any water or ice particles that have formed in the atmosphere or at the Earth’s surface as a result of condensation or sublimation. Water or ice particles blown from the ground into the atmosphere are also classed as hydrometeors. Some well-known hydrometeors are clouds, fog, rain, snow, hail, dew, rime, glaze, blowing snow, and blowing spray. Ice forms the frame work of the crystal and water/air fills the void. A snow cover is a collection of randomly arranged such crystals. As per WMO, classification of hydrometeors other than clouds and their symbols have given in table 1.

Evapotranspiration: It is the combined processes of movement of water from the earth surface as evaporation and plant surface as transpiration. It is also termed as consumptive use of water.

• Interception: It is the process, in which part of the precipitation before falling in to earth surface, caught by vegetations, structures, other surface modifications, and evaporated back to atmosphere. The part of the intercepted water by vegetations, which drip off the plant, leaves and joins the ground surface or surface flow is known as through fall.
• Infiltration: It is the processes in which water enters in to the ground surface.
• Percolation: Deep downward movement of water after saturation of soil is termed as percolation.

5. Rainfall

5.1. Rainfall analysis and characterization

For easy and useful interpretation and analysis of rainfall data, it has to present in proper manner. Most commonly used methods of rainfall data presentation are given below:

5.1.1. Mass curve

This is the graphical representation of cumulative rainfall over a period of time, plotted in a chronological order. Float type and weighing-bucket type rain gauges give the rainfall data in this form. From the mass curves of rainfall, useful information like the magnitude and duration of rainfall can be obtained. Intensity of rainfall at various time intervals for a given storm event can be obtained from slope of the curve.

5.1.2. Hyetograph

This is the graphical representation of rainfall intensity and time. The hyetograph is derived from the mass curve and represented as a bar chart. The hyetograph is required for development of a design storms to predict extreme floods. The area under a hyetograph represents the total precipitation received in that period.

5.1.3.Missing rainfall data estimation

Before use of rainfall data, the continuity and consistency of data should be checked first. The continuity of data may be broken due to missing of data, which may happen because of various reasons such as instrument failure or absence of the observer. Thus, it is often necessary to estimate the missing record using data from the neighbouring station. The methods are used for estimating the missing records are given below:

I.   simple arithmetic Method

II. Normal ratio Method

III. Modified normal ratio method

IV. Inverse distance method

For n stations, 1, 2, 3, …, n, the annual precipitation values are P1, P2, P3, …, Pn, respectively. Px is the missing annual precipitation (Px) at station x (not included in the above n stations). The normal annual precipitation N1, N2, N3, …, Ni at each of the above (n+1) stations including the station x is known.

I.Simple arithmetic average – The missing precipitation Px can be determined using simple arithmetic average, if the normal annual precipitation at various stations are within 10% of the normal precipitation at station, x, as follows:

II.Normal ratio method – If the normal precipitations vary considerably then Px is estimated by weighting the precipitation at various stations by the ratios of normal annual precipitation. The normal ration method gives Px as:

This method is based selecting n (n is usually 3) stations that are near and approximately evenly spaced around the station with the missing record.

As in inverse distance method the weighting is strictly based on distance, hence this method is not satisfactory for hilly regions.

5.2.Normal precipitation

It is the average value of precipitation at a particular date, month or year over a specified 30 year period. Thus, the term normal annual precipitation at any station is the average annual precipitation at that station based on a specified 30 years of record.

5.3.Frequency of point rainfall

Rainfall depth and intensity varies with space and time. Storms of high intensity and varying durations occur from time to time. The first step in designing engineering projects dealing with flood control, gully control etc. is to determine the probability of occurrence of a particular extreme rainfall.

This information is determined by the frequency analysis of point rainfall data.

Frequency analysis deals with the chance of occurrence of an event over a specified period of time. Suppose, P is the probability of occurrence of an event (rainfall) whose magnitude is equal to or in excess of a specified magnitude X. The recurrence interval (also known as return period) is related to P as follows:

T=1/p

There are two methods for performing frequency analysis: a) empirical method, b) analytical method. The exceedence probability of the event is obtained by the use of empirical formula, known as plotting position. Weibull formula is the most commonly used plotting position formula. In this method, first P and T are calculated for all the events in the series, and then the variation of rainfall magnitude is plotted against the corresponding T on semi-log or log-log paper. The rainfall magnitude for any recurrence interval can be determined by extrapolating the plot between magnitude and recurrence interval.

Empirical procedures can give good results for small extrapolations but the errors increased with the amount of extrapolation. For more accurate results, analytical methods using frequency factor are used.

5.4.  Spatial measurement of rainfall

Generally we measure precipitation, at a single point, which always not the representative of the amount of water fall over the entire area. As hydrological analysis requires information on the precipitation over a defined area, a network of point measurements is required. The network of precipitation measurement points can be converted to average value over the area by using a) Arithmetic mean method, b) Thiessen polygon method, and c) Isohyetal method.

5.4.1. Arithmetic mean method

In this method, the arithmetic mean of the precipitation for all stations within the area is computed. Since this method assigns equal weight to all stations irrespective of their relative location  and other factors, it should be used in area where rainfall is uniformly distributed. if P1, P2, …, Pi, Pn are rainfall values in a given period in N stations within the defined area, then mean precipitation over the area,  can be calculated as:

5.4.2. Thiessen polygon method

This is a graphical method, in which rainfall recorded at each station is given a weightage based on the relative areas of each measurement station in the Thiessen polygon network. The individual weights are multiplied by the station observation and the values are summed to obtain the average precipitation. This method is useful for areas, which are more or less plain and are of intermediate size (500 to 5000 km2). This method is also used when there are a few rain gauge stations compared to size.

Area of each polygon (Ai) is determined and the average precipitation is calculated using the following equation: Where, A is the total area.

5.4.3.Isohyetal method

This is a graphical technique which involves drawing estimated lines of equal depth of rainfall over an area based on point measurements. Then multiply the area between each contour by the average precipitation in the area to get the rainfall volume in the area. Sum these volumes to get the total rainfall volume, and then divide the total rainfall volume by the area of the watershed to get the average precipitation in the watershed.

6. Evapotranspiration: methods of measurement

1. Evaporation

Evaporation is the process by which water returns to the atmosphere as water vapour. Energy required to evaporate water depends on incoming solar radiation, reflectivity of the evaporative surface, and air and surface temperature.

Factors affecting evaporation

The factors that affect evaporation are:

• Wind: When wind speed is high it increases evaporation. However above certain wind velocity, further increase in wind velocity (critical wind speed) does no influence the evaporation. The critical wind speed is function of the size of the water surface.
• Temperature: The rate of evaporation increases with an increase in the water temperature. A strong correlation between evaporation rate and air temperature does not exist, though evaporation rate increases with increase in air temperature.
• Heat: Evaporation is more in summer in comparison to winter.
• Exposed surface area: evaporation rate increases with increase in exposed surface area. For instance, a wet cloth spread out dries faster than when folded.
• Humidity: Dryness assists evaporation, high humidity, less evaporation.
• Nature of the liquid: Rate of evaporation depends upon the type of liquid. The per cent reduction in evaporation approximately corresponds to the percentage increase in specific gravity. For example, petrol evaporates faster than water.
• Vapour pressure: If pressure is applied on the surface of a liquid, evaporation is hindered; consider, for example, the case of a pressure cooker.

6.1.1. Measurement of evaporation

Accurate estimation of evaporation is important in planning and operation of reservoirs, irrigation systems, and conservation of water. The amount of water evaporated from the water surface is estimated by a) using evaporimeters b) using empirical equations c) by analytical methods.

Evaporimeters: These are used for measuring evaporation, consist of water containing pan which are exposed to atmosphere and the loss of water by evaporation is measured at regular interval.

Most commonly used evaporimeters are USWB Class A pan evaporimeter, Piche evaporimeter or atmometer and Sunken Evaporation Pan.

6.2. Transpiration

Transpiration is the process in which liquid water contained in plant tissues removed to the atmosphere as vapour. Crops lose their water through stomata. Transpiration of plants depends on the energy supply, vapour pressure gradient and wind. Hence, radiation, air temperature, air humidity and wind terms should be considered when assessing transpiration. The soil water content and the ability of the soil to conduct water to the roots also determine the transpiration rate, as do water logging and soil water salinity. The transpiration rate is also influenced by crop characteristics, environmental aspects and cultivation practices. Different kinds of plants may have different transpiration rates.

6.3.Evapotranspiration (ET)

As evaporation and transpiration occur simultaneously, it is difficult to distinguish the two processes, thus in hydrology and irrigation practices, both the processes considered as one known as evapotranspiration. Evapotranspiration mostly influenced by the availability of water for a given set of atmospheric conditions. When sufficient moisture is available to meet the needs of vegetation fully covering the area, the resulting evapotranspiration is called potential evapotranspiration. The actual evapotranspiration occurring in a specific situation is called actual evapotranspiration. The evapotranspiration rate is normally expressed in millimetres (mm) per unit time. The rate expresses the amount of water lost from a cropped surface in units of water depth.

6.3.1. Measurement of evapotranspiration

Direct measurement of evapotranspiration can be done in four ways, viz., lysimeter technique, field experimental plot study, soil moisture depletion studies and water balance methods. Amonog the above, lysimeter is most accurate one. A lysimeter is a special water tight tank containing a block of soil and set in a field of growing plants. The plants grown in the lysimeter are the same as in the surrounding field. Evapotranspiration is estimated in terms of the amount of water required to maintain constant moisture conditions within the tank measured either volumetrically or gravimetrically through an arrangement made in the lysimeter. Lysimeter should be designed to accurately reproduce the soil conditions, moisture content, type and size of the vegetation of the surrounding area. They should be so buried that the soil is at the same level inside and outside the container. Though lysimeter gives good result, however it is time-consuming and expensive.

6.4. Estimation of evapotranspiration

The difficulty in direct measurement of evapotranspiration under field conditions resulted in use of different methods for estimation or prediction of evapotranspiration on the basis of climatological data. The approaches followed are to relate the magnitude and variation of evapotranspiration to one or more climatic factors (temperature, day length, humidity, wind, sunshine etc.). The most commonly used empirical formulae in estimating evapotranspiration are:

a) Blaney-Criddle Method

b) Thornthwaite Method

c) Hargreaves’ Method

d) FAO Penman-Monteith Method

Among the above mentioned methods, FAO Penman-Monteith Method is widely used method.

6.4.1. FAO Penman-Monteith method

The FAO Penman-Monteith method is used to estimate reference evapotranspiration. The equation is:

Where, ET0 = reference evapotranspiration [mm day-1] Rn   = net radiation at the crop surface [MJ m-2 day-1]

T = mean daily air temperature at 2 m height [°C] u2 = wind speed at 2 m height [m/s es   = saturation vapour pressure [kPa]

    S= potential maximum retention after runoff begins (in)

Ia= initial abstraction (in)

Initial abstraction (Ia) is all losses before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evaporation and infiltration. Ia is highly variable but generally is correlated with soil and cover parameters. Ia is approximated by

Ia= 0.2S.

By replacing Ia, above equation can be rewritten as

S is related to the soil and cover conditions of the watershed through the curve number (CN). CN has a range of 0 to 100 and S is related to CN by

The NRCS curve number is related to soil type, soil infiltration capability, land use, and the depth of the seasonal high water table. To account for different soils’ ability to infiltrate, NRCS has divided soils into four hydrologic soil groups (HSGs).

HSG Group A (low runoff potential): Soils with high infiltration rates even when thoroughly wetted. These consist chiefly of deep, well-drained sands and gravels. These soils have a high rate of water transmission (final infiltration rate greater than 0.30 in (7.6 mm) per hour).

HSG Group B: Soils with moderate infiltration rates when thoroughly wetted. These consist chiefly of soils that are moderately deep to deep, moderately well drained to well drained with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission (final infiltration rate of 0.15–0.30 in (3.8–7.6 mm) per hour).

HSG Group C: Soils with slow infiltration rates when thoroughly wetted. These consist chiefly of soils with a layer that impedes downward movement of water or soils with moderately fine to fine textures. These soils have a slow rate of water transmission (final infiltration rate 0.05–0.15 in (1.3–3.8 mm) per hour).

HSG Group D (high runoff potential): Soils with very slow infiltration rates when thoroughly wetted. These consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious materials. These soils have a very slow rate of water transmission (final infiltration rate less than 0.05 in (1.3 mm) per hour).

Stream flow is measured at gauging stations. Stream flow is unique among water cycle components in that it both spatially and temporally integrates surplus runoff and waters upstream within a catchment basin. Measurements are made by determining the discharge in each subsection of a channel cross section and summing the subsection discharges to obtain a total stream flow discharge.

8. Summary

• Hydrometeorology is a branch of meteorology and hydrology that studies the transfer of water and energy between the land surface and the lower atmosphere.
• Hydrometeors like clouds, fog, rain, snow, hail, dew, rime, glaze, blowing snow, and blowing spray are any water or ice particles that have formed in the atmosphere or at the Earth’s surface as a result of condensation or sublimation.
• Hydrological cycle is the study of water movement between the atmosphere, water bodies and earth; whereas, hydrology is a quantitative and qualitative study of water at different stages of the water cycle.
• The major components of hydrological cycles are precipitation, evaporation, transpiration, interception, infiltration and percolation. 
• Rainfall analysis and characterization is done by mass curve, hyetograph, depth-area relation and missing rainfall data estimation.
• Thiessen polygon method is applicable when there are a few rain gauge stations compared to size of the area.
• Curve number method is most accurate for estimation of runoff from rainfall.