3 Ground water Resources estimation and production

Objectives

Estimation method of Ground water.

Key Words

Hydrology, Porosity, Permeability, Ground a waterEstimation method in Monsoon Season and Non- Monsoon Season

Introduction

Water sustains the life of the earth system.It is indispensable for the well-being, welfare of human beings and their natural environment. Mismanagement, misuse, pollution,and scarcity of this life-sustaining resource pose a serious problem to the human being.It is growing threat to agriculture, food security, industrial development and the ecosystems on which they depend. Scarcity and poor quality of water can have a dramatic impact not only on agricultural production, but also on all aspects of everyday life. It is adding to the burden of women very often. There is Competition for fresh water in the world.The demand for fresh water has soared with the rapid growth of population, agriculture, urbanization, and industrialization during this century. Global water use has more than tripled while per capita use has increased by almost 50 percent since 1950. It has resulted in acute problem and competition among the various water users. The situation is aggravated because water resource is a common resource. water is still largely a common property resource (CPR) in the world.

Ground water resources are sources of water that are potentially useful in the critical situation. Uses of ground water have included agricultural, industrial, household, recreational and environmental activities. All living things require water to grow, flourish and reproduce.Water is essential to the life of the earth surface either organic or inorganic life. Population growth, urbanization, and expansion of the economic activities lead to increasing demand for water. There are various uses of water for different purposes. Domestic, industrial, irrigational, hyro-electric powers, thermal power, recreational are the major users of water resources on the earth surface. The quantity of quality water is the great concern of the water resources.

Water Resource

The concept of water resources is the quality of available water. It is not limited to its physical measure the flows, and reserve, but encompasses other more qualitative, environmental and socio-economic dimensions. However, estimation of ground water focuses on the physical and quantitative assessment of the water resource.

Types of water resources

Water resources are defined as the average flow of rivers and streamsas surface water and recharge of aquifers generated from precipitation as ground water. It distinguishes between the natural situationswithout human influence. It isan actual situation of natural flow in drains and aquifers. The estimation of the actual renewable water resources takes account of possible reductions in flow resulting from the abstraction of water in upstream.

Renewable: Renewable water resources are estimated on the basis of the natural water cycle.It represents the long-term average annual flow of surface water and groundwater.

Non-renewable water resources: Aquifers, which do not receive water from any source is non-renewable water resources. Groundwater, deep aquifers have a negligible rate of recharge can be considered non-renewable resource.

Natural renewable water resources

Natural renewable water resources are the total amount of a country’s surface water and groundwater, which are generated through the hydrological cycle. The amount is estimated on a yearly basis.

Hydrology

Darcy’s law along with the equation of conservation of mass is equivalent to the groundwater flow equation, is one of the basic relationships of hydrogeology.

“Hydrology is a subject of great importance for people and their environment which treats all phases of the earth’s water. Applied applications of hydrology are found in such way as the design, operation of hydraulic structures, water supply, wastewater treatment, disposal, irrigation, drainage, hydropower generation, flood control, navigation, erosion, salinity control, pollution, recreational use of water, fish cultivation and wildlife protection. The role of applied hydrology is to help analyze the problems involved in these tasks and to provide guidance for the planning and management of water resources”. (Chow et al. 1998)

Permeability:Permeability is defined as the property of soil which permits flow of water through it.Soil is highly pervious when water can flow through it easily. E.g. Gravels.In an impervious soil, the permeability is very low and water cannot easily flow through it. E.g. Clays.Rocks are impermeable. Permeability is a very important hydrological property of soils.

Application of Permeability: A knowledge of Permeability is essential for Settlement of building, Yield of wells, Seepage through and below the earth structures, Earth pressure and Uplift pressure under hydraulic structure.

Factors affecting permeability of Soils: The following factors affect the permeability of soilsParticle size. Properties of pore fluid, Void ratio of soil, Shape of particles, Structure of soil mass, Degree of saturation, absorbed water and Impurities in water.

Aquifer:An aquifer is an underground layer of water-bearing permeable rock, gravel, sand, or silt from which groundwater can be extracted using water well. The study of water flow in aquifer and the character of aquifer is called hydrogeology.

Source : National Geography Society

Character of aquifer

i. Properties: Porosity, specific yield, specific retention.

ii. Potential: Transmissivity, storability.

iii. Types: confined, unconfined.

iv. Hydraulic conductivity, Physical Laws controlling water transport

Porosity

Porosity is the quality of being porous, or full of tiny holes. Liquids go right through things that have porosity. Porosity stems from the Greek word poros for “pore,” which means “passage.”

The Principle of Ground Water Flow:

Porosity and effective porosity

Total porosity is the part of rock that’s void space;

nT = Vv/VT = (VT – Vs)/VT

Where Vv is Void volume, VT total volume and Vs is solid volume.

Void ratio (e )=Vv/Vs

Types of Porosity;

Primary Porosity: It is original in rock. Primary porosity is range from 26 percent to 47 percent in different arrangements and in ideal spheres.

Secondary Porosity: It is in fracture or solution porosity.

Total Porosity: It is an amount of pore space. It does not require pore connection.

Effective Porosity: It is percentage of interconnected pore space available for groundwater flow.

Porosity is one order of magnitude smaller than total porosity.

Porosity in Sediments and Rocks: It is depending on grain size, particle shape, arrangement, and actual values of porosity ranges from zero and near zero to more than 60 percent. The smaller the particle size and the higher the porosity is in the sedimentary rocks.

Transmissivity: The amount of water that can be transmitted horizontally through a unit width by the full saturated thickness of the aquifer under a hydraulic gradient of 1.

Steady State Flow to Well

Simplifying by assuming K = constant in all dimensionsand assuming that

Transmissivity T = Kb and

Q = flow rate to well at point (x,y) yields

Darcy & Seepage Velocity : Darcy velocity is a fictitious velocity since it assumes that flow occurs across the entire cross-section of the soil sample. Flow actually takes place only through interconnected pore channels.

h1 = the height of the inlet head,

h2 = the height of the outlet head,

i = the hydraulic gradient,

L = the path length of the flow.

The flow in unsaturated zones is vertical due to gravity. The water in saturated zones has a total potential of gravity and the pressure component. The flow in an unsaturated formation differs from a saturated flow in terms of the sensitivity of hydraulic conductivity. Hydraulic conductivity in unsaturated formations varies with the moisture content, and these two vary with the hydraulic pressure. These principles are used in estimation of precipitation recharge in unsaturated formations. The pressure arises from the capillary forces that hold water in the interstitial pore spaces in an unsaturated flow. This capillary force is always negative below the atmospheric pressure.

Ground Water Resource Estimation Methodology

Ground Water Balance Equation

The ground water level fluctuation method is based on the application of ground water balance equation.This is in general terms as follows for any specified period,

Input -Output = Storage increase

Equations of Groundwater Flow

Description of ground water flow is based on Darcy’s Law Continuity Equation– It describes conservation of fluid mass during flow through a porous medium; results in a partial differential equation of flow.

Laplace’s Equation;

Mass In – Mass Out = Change in Storage

The terms input and output are used in the general sense referring to all components of ground water balance in the above equation.These are either input to the unit or output from the unit of ground water system taken up for resource assessment.Input refers to recharge from rainfall and other sources and subsurface inflow into the unit. Output refers to ground water draft, ground water evapotranspiration, and base flow to streams and subsurface outflow from the unit. It is desirable to apply the equation separately for different seasons, such as monsoon and non-monsoon seasons.

From ground water assessment point of view, The right side term in eqn. 1, namely storage increase (positive for storage increase, negative for storage decrease), is given as a function of the ground water level change and specific yield. Hence ground water level measurements at the beginning and end of the season form necessary input for the estimation of storage change.

The input and output terms in eqn. 1 include subsurface inflow and outflow components across the boundary of the unit, which depend on the transmissivity and hydraulic gradient. It is advantageous to adopt the unit for ground water assessment as basin and watershed, as the inflow-outflow across these boundaries may be taken as negligible.

Delineation of watershed for ground water recharge

A hydrogeological boundary of watershed is an appropriate hydrological unit for ground water resource estimation. The hydrogeological and hydrological units coincide in hard rock areas.It is not in alluvial areas where the aquifers traverse the basin boundaries.Assessing the ground water on watershed as a unit is more desirable in hard rock areas which occupy about 2/3rd area of the country.The development unit is either a block or a taluka in many states of India. The present methodology of assessing the ground water potential on block or taluka-wise basis may continue in case of alluvial areas. where it is difficult to identify watershed considering the trans-boundary aquifer system. The predominant hydrogeology of the unit is to be considered for purposes of classification into alluvial or hard rock areas.Alluvial patches occurring in predominantly hard rock area is considered as part of the watershed unit. However, the state ground water departments areendeavoring to demarcate watershed as a unit for assessment.

Ground water level fluctuation method

The ground water level fluctuation method is used for recharge assessment in the monsoon season.Recharge in the non-monsoon season is a small component and may be estimated empirically as described subsequently for non-command areas.Two alternate approaches are possible in the ground water level fluctuation method;

a. Estimate specific yield from long duration pumping tests for the particular hydrogeological area. To estimate recharge of the ground water, use value of specific yield in the ground water balance equation for the monsoon season. This method is suitable for alluvial areas and hard rock areas when data of base flow in the dry season is not available.

b. Ground water balance equation is applied separately for the dry season to estimate specific yield.Use value of specific yield in the ground water balance equation for the monsoon season to estimate recharge. This method providesa reliable assessment of recharge in hard rock areas where adequate data of base flow in the dry season is available. It is also useful in case of the base flow in the dry season is practically negligible.

Case A: Monsoon Season

Specific yield value is obtained from long duration pumping tests or from norms for different hydrogeological areas in this approach. It is necessary to have test sufficiently long duration. The tests duration is minimum pumping of 16 hrs.Proper assessment of specific yield value is difficult in using pumping tests to obtain specific yield value.

In situations where specific yield value cannot be estimated by other means. the norms for estimation of recharge aregiven in table-1.

Table-1 Norms for Specific Yield

Ground Water Recharge Estimation in the monsoon season:

The water level fluctuation method is applied for the monsoon season to estimate the recharge. The ground water balance equation for the monsoon season in non-command areas is given by,

RG – DG – B + IS + I = S —————————————————– (1)
Where;

RG = gross recharge due to rainfall and other sources including recycled water DG = gross ground water draft

B = base flow into streams from the area

IS = recharge from streams into ground water body

I = net ground water inflow into the area across the boundary(inflow – outflow)

S = ground water storage increase

All quantities in above equation refer to the monsoon seasononly.

If the area is under watershed, the net ground water inflow term, I may be taken as zero in above equation. If there is inflow and outflow across the boundary, the net inflow may be calculated using Darcy law, by delineating the inflow and outflow sections of the boundary.The calculation also requires estimate of transmissivity and hydraulic gradient across the inflow and outflow sections besides such delineation. These calculations are most conveniently done in a computer model. A need therefore arises to consciously employ computer based distributed parameter system approach using techniques like finite difference, finite element and boundary integral equation methods. The present ground water assessment as prescribed in these recommendations, the net inflow term, I may be dropped.

There are similar difficulties in estimating the base flow and recharge from streams. After deleting net inflow and base flow terms in equation (1) the resultant recharge refers to the possible recharge under the present status of ground water development in the area. This possible recharge is the gross recharge minus the natural discharges in the area during the monsoon season. To signify this, the RG term in equation (1) is rewritten as R. Equation (1) is now rewritten as,

R = S + DG ———————————————————————–(2)

Where;

R= possible recharge, which is gross recharge minus the naturaldischarges in the area in the monsoon season (RG – B + I +IS)

Substituting the expression for storage increase S in terms of water level fluctuation and specific yield becomes,

R = h x Sy x A + DG—————————————————————-(3)

Where;

h = rise in water level in the monsoon season

A = area for estimation of recharge

Sy = specific yield

The recharge calculated from equation (3) gives the available recharge from rainfall and other sources for monsoon season. The recharge from other sources may be recharge from recycled water from ground water irrigation, recharge from tanks, ponds and recharge from water conservation structures in non-command area including check dams, percolation tanks, canal bunds etc. The recharge from rainfall is given by,

Rrf = R – Rgw – Rwc–Rt=hxSy x A + DG – Rgw – Rwc – Rt———————— (4)

where

Rrf = recharge from rainfall

Rgw = recharge from ground water irrigation in the area

Rwc = recharge from water conservation structures

Rt = Recharge from tanks and ponds

There are different norms for estimation of recharge from ground water irrigation (Rgw), recharge from water conservation structures (Rwc) and recharge from tanks and ponds (Rt) may be made based on the norms given in table-1. The recharge from rainfall estimated as per equation (4) is for the particular monsoon season.

Estimation of normal recharge during monsoon season

It is procedure for normalisation of this recharge for estimating recharge corresponding to the normal monsoon rainfall. Two methods are possible for the normalisation procedure.

a. The first method is based on a linear relationship between recharge and rainfall ;,

R = a r Where;

R = rainfall recharge during monsoon season r = monsoon season rainfall

a = a constant

b. The second method is also based on a linear relation between recharge and rainfall. However linear relationship is ;

R = ar + b

where;

R = rainfall recharge

r = rainfall

a and b = constants

Case-B : Dry Season

In this approach, the specific yield is estimated from ground water balance in the dry season.Recharge is estimated based on specific yield value from ground water balance in the monsoon season.This approach is suitable in hard rock areas.If data of base flow in the dry season is available. Base flow in the dry season is practically zero.

Ignore the net inflow term due to subsurface flow and assume that the recharge from rainfall during the dry season is practically zero, the ground water balance in the dry season is;,

h x Sy x A = DG – Rgw + B —————————————————————- (5)

where

h = decrease in ground water level

DG = gross ground water draft

Rgw = recharge recycled from ground water irrigation B = base flow from the area

All quantities in equation (5) refer to the dry seasononly. The estimation of recharge from ground water irrigation (Rgw) may be made based on the norms of type of crop paddy, non-paddy and the depth of water table below ground level. Specific yield value is determined from the water level fluctuation data in the dry season. The recharge in the monsoon season can be calculated from equation- 3.

Ground Water Development

Stage of ground water development

The stage of ground water development is;

Stage of ground water development in percent = Existing gross ground water draft for all uses / Net annual ground water availability X 100

The existing gross ground water draft for all uses refers to the total of existing gross ground water draft for irrigation and other purposes. The stage of ground water development is obtained separately for non-command areas and command areas.

Recharge assessment of rainfall infiltration factor

In case adequate data of ground water levels are not available. Recharge can be estimated based on the rainfall infiltration factor method. Recharge from rainfall in monsoon season is;

Rrf = f x A x Normal rainfall in monsoon season Where;

f = given rainfall infiltration factor

A = area of estimation for recharge

The same recharge factor is used for both monsoon and non-monsoon rainfall. If the normal rainfall during the non-monsoon season is less than 10 percent; the recharge due to non-monsoon rainfall may be taken as zero in this case.

Ground water level fluctuation method

Indirect methods widely used in India for estimating groundwater recharge. It is simple and direct, the method makes use of readily available data on rainfall, runoff and water levels. The ground water level fluctuation method is applied for estimating the recharge in the monsoon season for the commandarea. Recharge from other sources include seepage from canals, return flow from surface water irrigation and ground water irrigation, storage tanks and ponds, and water conservation structures in command area. The recharge from rainfall is as follows;

Rrf = h x Sy x A + DG – Rc – Rsw – Rt – Rgw–Rwc Where;

Rrf = recharge from rainfall

Rc = recharge due to seepage from canals

Rsw = recharge from surface water irrigation

Rt = recharge from storage tanks and ponds

Rgw = recharge from ground water irrigation

Rwc = recharge from water conservation structures

DG = gross draft

h = rise in ground water level

A = area of the command area

Sy = specific yield

All quantitiesarereferring to the monsoon season only. The recharge quantities from other sources may be zero for particular command area. It is noticed that the net ground water inflow across the boundaries has been ignored. In such cases a freedom is given for including the component of net ground water flow across the boundaries. This can be estimated as the product of gradient of ground water flow, transmissivity of the aquifer and the length across the flow.

Conclusion

Ground water recharge is explained as the process of flow of water in aquifers. The amount of water flow in or flows through in the porous sub-soil is increases by natural or artificial means. The amount of water extracted from an aquifer without causing depletion is primarily dependent upon the ground water recharge. Rainfall is the prime source ofground water recharge and moisture in the soil water system. There are other sources include recharge from rivers, streams, and irrigation water. The amount of moisture reach the water table is defined as natural ground water recharge.It is depends on the rate and duration of rainfall, the subsequent conditions at the upper boundary, the antecedent soil moisture conditions, the water table depth and the soil type.The total annual recharge is obtained as the sum of recharge in the monsoon season and recharge in the non-monsoon season, where in each season, the recharge comprises of recharge from rainfall and recharge from other sources.

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