What is 'storage'?What are the factors influencing the storage?how does it affect the environmental balance of the hydrological cycle?

"storage' is a hydrological term referring to reservoirs of water contained within the hydrological cycle.These include surface water storage ,soil water storage and ground water storage.

Aquifers serve as reservoirs for ground water storage which is controlled by factors like-porosity,specific retention and specific yeild.The amount of water in storage can be determined by the water balance equation:-

The mechanism of the storage changes in the confined and unconfined aquifers.

There are certain aquifer factors which influence storage:-

1)Porosity;

2)Specific yeild and specific retention;

3)Storage changes in an unconfined aquifer;

4)Storage changes in a confined aquifer.

1)Porosity:-It is expressed as percentage of the total volume of a rock or the soil which is represented by it's interstices or the voids.the formula for the measuring of soil porosity is :-

The value obtained from the equation is multiplied with 100 and expressed as percentage.

Total porosity is thelow in sedimentary rocks because cement partially or wholly fills pores between grains.total porosity is low in igneous and metamorphic rocks because minerals crystallize together at their formation well round the particles pack more tightly than angular particles and thus have lower porosity.,very low porosity like those in low storage while high porosity indicates high storage.

Effective porosity which is the volume percentage of a rock or sediment sample that consists of the interconnected pores through which water flows.Good aquifers like those of the sandstone has low to medium porosities.Although porosity determines holding capacity of a saturated medium but that doesn't imply all the water will be available for the movement in the hydrological cycle.

Showing typical ranges of porosity
Material	Range of porosity(%)
Soils	30-65
Clay	35-60
Karst Limestone	05-50
Crystaline rocks	00-10

2)Specific yield and specific retention:-

Specific yield is defined as the volume of the water that can freely drain from a saturated rock/soil under gravity and is expressed as total volume of the aquifer,and is reliably measured by the well pumping test.

Fine grained materials yield little water.Coarse grained materials yield much water and serve as aquifers. Specific retention of a soil or rock is the ratio of the volume of the water it will retain after saturation against the force of gravity to it’s own volume.

Sr=Wr/V where,Wr=volume occupied by water(retained);V=bulk volume of the soil or rock.

Showing specific yield
Material	Yeild(%)
Clay	3
Schist	26
Limestone	14

3)Storage change:-

Storage change is referred as the change in the storage at any one time and expressed by water-balance equation.Coefficient of storage is defined as the volume of water that an aquifer takes into or releases from the storage per unit surface area of the aquifer’s per unit change in head.Storage coefficient of the confined aquifers tend to be significantly smaller than those of the unconfined aquifers.

STORAGE CHANGES IN AN UNCONFINED AQUIFERS:-

1)seasonal fluctuations:-reflects seasonal changes in storage and water availability.normal climatically determined ground water regime is modified by artificial factors like irrigation and drainage in some regions .

2)Secular fluctuations:-abstraction for the industrial,agricultural and domestic purposes which result in the incursion of inferior quality,saline ground water from adjacent aquifers or direct seepage from the oceans into coastal aquifers.

3)Short term fluctuations:-In many coastal and estuarine aquifers fluctuations are related to the tidal movements,if there is simple sea level variation with the harmonic motion, a train of sinusoidal waves is propagated in land from the submarine outcrop of an aquifer.In the valley bottom area fluctuations occur during the hottest part of the day when discharge as evaporation from ground water exceeds the rate of inflow from surrounding higher areas.

STORAGE CHANGES IN CONFINED AQUIFERS:-

If the confined aquifers is perfectly elastic such that discharge equals recharge potentiometric head is restored.Aquifers with high clay content may undergo permanent compaction and the porosity loss causing reduced potentiometric head.

Any variation of the surface fluctuations for the given variation of the potentiometric level,the change of the ground water storage in a confined aquifer will be small compared to an potetiometric surface in a confined aquifer is smoother than the water table of the unconfined aquifer because local head changes are more rapidly propagated as recharge/discharge wave.

Conclusion:-It can be said that storage change and it’s factors contribute to the environmental balance of hydrological cycle.for example:-excess discharge over recharge in confined aquifers may result in subsidence of the overlying ground surfaces.

Many rivers may go dry during summer if ground water storage is disturbed.Low ground water storage indicating low saturation of soil horizon will in turn affect evaporation and precipitation it will also affect plant growth.