ANALYSIS OF CLIMATE PARAMETYERS

Normal rainfall

Normal is generally the long-term averages of the rainfall in areas. These are computed and published by the India Meteorological Department from time to time. The current normal are for the period 1961-1990. Rainfall whether it is annual or seasonal is generally compared with long term averages (also called normal) to know the pattern and behavior. If the percent deviation of actual rainfall from long term average is –19% to +19% it is normal, -20% to –59% it is deficient, >-60 it is scanty, 20% to 59% it is excess and >60% it is large excess.

Drought Analysis

The best way to define an area, as drought prone is to calculate the coefficient of variation (CV). If the CV value of the annual or seasonal rainfall is more than 30% or the CV value of the monthly rainfall is more than 50% then the rainfall of the area is called to be erratic and drought prone.

Trend Analysis

Regression: analysis is widely used ground water studies like pumping test analysis, resource estimation, correlation between rainfall and water level etc. The trend of the rainfall is computed using the statistical technique called regression analysis. The method used is least square method. In this line of best fit is fitted to the data representing either a time series data or two sets of data like rainfall and water level.

It is basically used to for the estimation of the value of the dependant variable like runoff with the known value of the independent variable like rainfall. The only condition is that the correlation coefficient (degree of association) between the variable should be significant statistically, in general it should be more than 90 %. The data is also graphically plotted and the best-fit line is drawn.

Evapotranspiration

Other important parameters like Evapotranspiration is either empirically estimated using filed or laboratory methods or theoretically computed using formulas given by C.W. Thornthwaite. This method is popular as it is easy to calculate and requires data of few climatic parameters. This method is also used to compute the monthly or daily soil moisture budgeting, which is useful in estimating the soil moisture surplus and deficit.

Runoff

Runoff in catchments or in a basin is another important parameter of interest in the analysis of the ground water studies. Runoff in catchments is generated as a result of rainfall. Hence it is obvious that there exists a mathematical relation between the two, which can be found with regression analysis. Estimation of runoff in catchments is generally done by steam gauge measurement in a stream or as river in the area.

Base flow

A river is called influent if it recharges the ground water body and is called effluent if it gets recharged from the ground water body. The flow in the river in the lean period is called base flow. Estimation of this is important as it gives estimation of the ground water flow into the river. This is also estimated by analyzing the discharge hydrograph.

RAINFALL ANALYSIS

A good network of rain gauges is a must for proper assessment of (ground) water resources. The planning of network primarily depends on physical factor, which affect hydrology such as topography, morphology and use and soil type. In an area where heavy rainfall characterised by extreme rainfall variability, denser should be the rain gauge network.

Rain Gauge Network

As per the IS 4987-1968, the rain gauge network density is as follows. In plain areas the density is one rain gauge for every 520 sq.kms, area of moderate elevation (average being 1000 m.s.l) the density is one rain gauge in 260 to 390 sq km where as in hilly area the density should be one rain gauge for every 130 sq.kms. Based on this criteria, we can check the basin have adequate number of rain gauges by the following formula
N = Cv/ P

Where N -optimum number of rain gauges stations, Cv -coefficient of variability in % and P -desired degree of percentage error. For micro level studies of small watersheds the error may not exceed 5% and for routine hydrogeological in investigations error of estimate should not exceed 10%

Normal of Data

Length of rainfall data records to be considered for analysis is very important. The length or period of record needed to achieve stability varies between regions to regions. For our Indian conditions it is observed, that rainfall data of 30 years in adequate. The correct normal period is 1961-90 and next normal period is 1971-2000.

Double Mass Curve

For proper ground water resources evaluation consistent rainfall data is essential. Due to change of location of rain gauge or its exposure condition, the data becomes inconsistent and this can be rectified by double mass curve method. Double mass curve is nothing but plotting of cumulative rainfall of the gage in question against average cumulative rainfall of a number of surroundings gauges. In double mass curve, the inconsistency will represent by a change in slope of the straight plot. Here the values of previous period i.e. before change in slope are adjusted to the current period i.e. after change of slope by adjustment to the current period i.e. after change of slope by adjustment factor where the adjustment is the ratio of the slope after change and slope before change.

Moving Averages

A Change in rainfall trend may be real cause by climatic conditions or damage to the instrument or change of location. Evidence of real trends may be ascertained from the study of progressive long-term average using 3 years or 5 years moving averages. For 3 year moving average the rainfall is averaged over successive 3 years periods. That means the first average is obtained for year 1,2,3. The second one for years 2,3,5 etc. In order to facilitate the plotting, the selection of moving average is odd period.

Supplementing Data

In order to supplement incomplete rainfall records, there are 3 methods.

In first method, it is assumed that the ratio of monthly or annual rainfall of two adjacent gauges is equal to the ratio of the normal rainfalls for the same period of the two gauges.

It is expressed as


Here x is the rain gauge whose records are to be interpolated and y nearest rain gauge. Rx is the rainfall at gauge x and Ry is the rainfall at gauge y. Nx and Ny are normal rainfall at x and y respectively.

In second method at least three rain gauges with continuous records, as close to and as evenly spread around the gauge with missing records, as possible are chosen. If the normal rainfall at each of the gauge is within 10% of that for the gauge with missing records, then the arithmetic mean of the rainfall, of the three surrounding gauges is used as that for the gauge in question. If the percentage is more than 10 the

Where
Nx, Na, Nb and Nc - the normal rainfall
Rx, Ra, Rb and Rc - the interpolated value of the gauge in question and the actual of three gauges.

Where as in the third method, the area around the station in question is divided into 4 quadrants, North, South, East and West. The weight of each gauge in this case is given by the reciprocal of the square of the distance of the gauge from the central station and weighted average for the central station is given by



Rn, Rs, Re and Rw - the rainfall at north, south, east and west quadrant respectively.
Xn, Xs, Xe, Xw - the distance of these four gauges for the central gauge.
Rainfall Average

The average of rainfall is usually obtained by any of the three methods: 1.) Arithmetic mean, 2.)Theissen polygon and 3.) Isohyetal.

Arithmetic method gives equal weights to all rain gauges, and it is quiet easy and it yields fairly accurate results if the rain gauge is uniformly distributed. The polygon method is laborious. But once weights of each rain gauge are completed, it gives results quickly. This method takes care of non-uniform distribution of rain gauges. The third method, isohyetal method overcomes most of the deficiencies of other two. The accuracy of the method depends on the skill with which the isohyets are drawn. It is reasonable to reply on polygon method for averages of plain areas and the isohyetal method for mountain areas.

Inadequacy of Rainfall

In general the arid areas receives animal rainfall of 40 cms are perceptually drought stricken and in the case semiarid areas canal rainfall between 40 and 75 cms are considered to be drought prone. According to Irrigation commission the drought is when annual rainfall is less than 75% of the drought is when drought as defined above, has occurred in 20% or more of the year that area is called as drought prone area in the case where the percentage number of year is 40 or more the area is said to be perpetually drought stricken.

Agriculture commission has classified drought, based on percentage negative departures of animal rainfall from normal annual rainfall.

On this basis

Mid drought - if the departure is up to 25%

Normal drought – if the departure is 26 to 50%

Severe drought - above 50%

Indian Meteorological Department has classified rainfall based on departures, If rainfall departure is between +19 and –19 then it is cased to be normal, -20 and –59 indicates deficient and above –60 - scanty condition.

RUNOFF ANALYSIS

Hydrology is concerned with waters of earth, their occurrence circulation and distribution, their chemical of physical properties and their reaction with environmental including their relation to living things (Eagles, 1970). Runoff occurs after the rate of precipitation exceeds the infiltration capacity for sufficient time to fulfill the demands of depression and establish an initial quantity of surface detention. Stream flow and river discharge often mean runoff. The runoff pattern of a given basin or area depends up on two sets of factors

1. Climatic factors: These are type of ppt rainfall intensity, duration, distribution, direction of rainfall.
2. Physical characteristics of drainage basin: This includes land use, type of soil, area shape slope type of drainage etc
Water Balance

Study of the water balance is application in hydrogeology of the principle of conservation of mass often referred to as the continuity equation. In general, the use of water balance technique implies measurements of both storages of fluxes of water.

Precipitation

Precipitation includes all forms of water deposited on earth’s surface and derived from atmospheric vapour. The principle forms are mist, rain, hail, sleet, and snow. In majority of cases, precipitation is synonymous with rainfall. This can be measured or computed by f following three methods,

1. Arithmetic mean
2. Thiessen mean and
3. Isohyetal method

Infiltration

Infiltration is the process where by water enters surface strata of me soil and moves downward toward the water table. The maximum rate at which a soil in any given condition is capable of absorbing water is calls its infiltration capacity or f-capacity. This is determined by ring method.

Evaporation

Evaporation is the process by which a liquid or a solid is changed in to vapor. The rate of evaporation depends on the pressure gradient. If the vapour pressure in the air above is less than that in the film immediately adjacent to the water surface vapour near the water surface will be dispensed through diffusion, convection and wind action.

Transpiration

Transpiration is the process where the moisture that has circulated through the plant returns structure to the atmosphere. The rate of transpiration for a given plant varies with crop growth from day to night, temperature, sunlight moisture available etc.