16 Water Balance and Drought

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Objectives

  • Concept of Water and Heat Balance, Water Balance of Surface/ Subsurface and Atmosphere, Process of Ground Water Out Flow, Meaning and Types of Drought, Impacts
    of Drought

   Keywords

  • Water Balance, Heat Budget, Hydrological Cycle, Radiant Energy, Evaporation, Transpiration, Runoff , Soil Moisture, Ground water, Hydrological Drought

    Introduction

 

The water dynamics like occurrence, amount and circulation on Earth have been always of interest to mankind. As we know that our climate system is largely regulated by the global energy and water balance and their spatial and temporal variations, which involves the flow of energy and water within the climate system and their exchanges with outer space and the surface.

 

Historically the overall picture and the details about water balance was first marked by Copernicus (1543), he had calculated the mass balances of the Solar System and considering the revolutions of cosmic bodies. In the present scenario the water balance of Earth extended immensely importance due to lack of drinking water, deteriorating water quality and declining water resource and groundwater table. We all are experiencing negative impacts of changing weather conditions (changes in heat budget, hydrological cycle and atmospheric circulations) and on some extent we are also responsible for these unwanted changes. Various studies are going on global circulation of water to evaluate the severity of the currently perceived water crisis situation, and predictions and consequences of climate change. The hydrological cycle forms the most dynamic part of the overall water circulation of planet Earth. While it is the most important water cycle in our daily life, other water cycles which operate on geological time scales are also important in visualizing a complete water perspective of our planet.

 

Concept of Water Balance

 

The word ‘water balance’ was first of all used in 1944 by the famous meteorological and climate scholar C. Warren Thornthwaite (1899-1963), who meant that it is a balance between precipitation, water obtained by melting of snow and evaporation, groundwater recharging and surface flow of water. The earth’s water budget is driven by precipitation and evaporation. Air temperature, precipitation, soil moisture, and evapotranspiration are related through the balance of incoming and outgoing energy, in combination with water at the earth’s surface. The incoming energy directed towards the surface is comprised of shortwave radiation from the sun and longwave radiation from the atmosphere. The outgoing energy from the earth surface is comprised of longwave radiation and heat that is associated with latent heat andsensible heat (Fig 1).

 

Fig.1: Global Heat Energy Budget

 

Fig.1: Global Heat Energy Budget

 

Fig.2: Hydrological Cycle

 

Similarly the water balance is the ratio between water inflow and outflow estimated for different space and time scales, i.e. for the Earth as a whole, for oceans, continents, countries, natural-economic regions, and river basins, for a long-term period or for particular years and seasons (Fig 2). As we can see in the Table 1 that the oceans and seas are having highest volume of water on the other hand the volume of water in the Atmosphere is much lower than oceans despite having large area. The residence period of oceans and ground water is highest among all the water bodies, followed by icecaps and glaciers. Thus oceans, ground water and glaciers have important role in upholding water balance on earth surface.

 

Table 1. Estimate of the World Water Balance

For each continent, evaporation is equal to the difference between precipitation and run-off. The ratio of evaporation to run-off differs greatly on various continents. In Australia evaporation is close to precipitation. In all other continents except Africa, evaporation is less than about 66% of the sum of precipitation. The amount of river runoff into the oceans from the land is approximately same as the amount of evaporation and precipitation from the oceans. For the individual ocean, evaporation is equal to the sum of the river run-off into it and horizontal transfer of water from other oceans through global oceanic circulation processes. For Atlantic Ocean sum of precipitation and run-off is less than the magnitude of evaporation on the other hand in Arctic Ocean evaporation is substantially smaller than the sum of precipitation and river run-off (Table 2).

 

Table-2: Precipitation, evaporation and run-off on continents & oceans.

 

The inflow of water vapour into the atmosphere from evaporation may be both larger and smaller in different latitudinal zones. The difference between precipitation and evaporation is equal to the difference between the inflow of water vapour into the atmosphere and the outflow resulting from the horizontal air movement. The components of continental and oceanic water balance are not constant and change as a result of climatic fluctuations and other factors (Fig 3). Studies have shown that in the 20th century, level of World Ocean has increased by about 15 cm while reserves of subsoil water on land and volume of water in many lakes have declined. Though sources of water contributing to rise in water level in World Ocean are not absolutely clear, melting of ice covers, loss of groundwater and decrease of water in lakes on land are responsible for the increase in volume of oceanic water.

 

Fig.3: Sea Level Rise due to temperature Warming

 

Water Balance of Surface/Subsurface

 

Water balance associated with determining spatial climate features, typical landscapes, possible water management and land use. Precipitation, evaporation, river runoff and ground water outflow not drained by river systems are basic components of water balance. For the assessment of heat energy and water balance accuracy in data on precipitation, evaporation and runoff (surface and subsurface) is very important.

 

Water balance equation for land surface may be given as:

 

P = E + SR + M

where, P = precipitation; E = evaporation at Earth’s surface which is equal to the difference between evaporation and condensation at Earth’s surface; SR = surface runoff; M = flow of moisture from Earth’s surface to deeper layers.

 

Evaporation is one of the major processes of transformation of solar energy at Earth’s surface and it very much affects the yearly run-off. Thus, run-off normal and coefficient of run-off are linked to principal components of energy balance. Average total evaporation from land surface depends on the quantity of precipitation and inflow of solar energy. Evaporation increases with increase in radiation balance. If soil is dry (in desert area) total precipitation is caught by molecular forces on soil particles and eventually expended on evaporation. In such conditions run-off coefficient will be zero. Thus, the average dryness will rise with increase in radiation and decrease in precipitation.

 

Latent and sensible heat: are types of energy released or absorbed in the Latent heat is related to changes in phase between liquids, gases, and solids. Sensible heat is related to changes in temperature of a gas or object with no change in phase.

 

Important indicative characteristics of hydrological regime on land are:

 

(a) Run-off normal: It is the volume of water flowing off on the average during a year from a unit of land surface in form of various horizontal flows;

   

(b) Coefficient of run-off: It is the ratio of run- normal to total yearly

 

(c) Radiation index of dryness: It is the ratio of the total radiation energy received to total energy expended on precipitation.

 

 Points to Remember

 

For sufficiently high value of total precipitation and sufficiently small value of total inflow of radiation, a state of full moistening of the upper layers of soil will be achieved. In such cases, the maximum portion of heat energy available from all sources will be expended on evaporation. The value of this expenditure may be calculated by considering the valve nature of the latent heat exchange between underlying surface and the atmosphere.

 

Experimental studies have shown that turbulent heat conductivity of lower layers of atmosphere depends substantially on the direction of vertical turbulent heat flow. If the direction of this flow is from Earth to atmosphere, values of turbulent heat flow become higher due to increased turbulent mixing. If direction of turbulent heat flow is from atmosphere to Earth, inverting temperature distribution reduces exchange intensity and turbulent heat flow becomes small. As a result of the valve effect, average turbulent heat moves from Earth’s surface to atmosphere in nearly all climatic zones of land i.e. yearly sums of turbulent heat flows are positive. Thus yearly turbulent heat flows cannot produce substantial inflows of energy to underlying surface and heat expenditure on evaporation are compensated only by the radiation balance. The magnitude of possible evaporation at a locality is determined by the radiation balance which is different in conditions of sufficient and insufficient moisture.

   

Contribution of Plants and Vegetation in Balancing Water

 

The atmosphere receives a considerable amount of moisture through transpiration of plants which usually represents several tens percent of total evaporation.Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. It is found that about 10 percent of the moisture in the atmosphere is released by plants through transpiration remaining 90 percent come through evaporation from oceans and other water bodies. Therefore, transpiration may exert a considerable influence on the atmospheric water exchange. The molecules of water vapour of external origin and local origin are mixed in the atmosphere during turbulent exchange. Therefore, the ratio of total precipitation derived from external water vapour and that derived from local water vapour is equal. The precipitation in an area also depends on the plant cover which influences the quantity of local evaporation. It has been observed that when mossy swamps are drained and a forest cover develops in such area, total evaporation appears to increase. The reverse effect is observed when lowland grassy swamps are drained. The influence of evaporation on precipitation depends on the size of area in which evaporation occurs.

Fig 4: Plant, Soil Water Cycle

 

Evaporated water cools the earth as a whole not only the local area of evaporation. Evaporation of water from trees and lakes could have a cooling effect on the entire atmosphere. Vegetation influences both albedo of the earth and the amount of water vapour and carbon dioxide in the air. All types of plants play a role in both the water cycle and the earth’s energy balance. Plants also produce their own micro-weather by controlling the humidity and temperature of their immediatesurroundings.In moist climates, especially warm and wet climates, the potential for evapotranspiration is very large like equatorial regions, here the value of bowen ratios (contributions of latent heat to net radiation) is very small. In dry regions, lack of water to evaporate leads to high bowen ratio.The significance of the Bowen ratio is that it gives us an indication of the relative partitioning of net radiation in a region.

 

Water Balance of Atmosphere

 

At a particular place in the atmosphere, the water is brought by evaporation from the Earth’s surface and is removed by precipitation or air currents and horizontal turbulent exchange. The water balance equation for atmosphere is, therefore, obtained by summing all the categories of inflow and expenditure of moisture within a vertical column passing through the atmosphere and is given as:

 

E = r + Ca + ba

Where, Ca = quantity of moisture received or lost by vertical column due to air currents and horizontal turbulent exchange; ba = change in quantity of water in the column.

 

Water Balance of Water Bodies

 

The same equation which is used for surface water balance can be used for water balance of water bodies. Here runoff will describe the total redistribution of water along the horizontal plane during the time interval under consideration both within the water body itself and in layers of underlying soil. Similarly, for a closed water body it will also be equal to overall change in the water content both within the body of water itself and in underlying layers in these cases there will be perceptible changes in moisture content.

 

Contribution of local evaporation to total precipitation- The principal source of water vapour for precipitation over continents is advective water vapour, which originates as oceanic evaporation (O.A Drozdov). The role of local evaporation is modest in total volume of precipitation, especially for the territories of dimensions less than 1 million square kilometers. The indirect influence of local evaporation on total precipitation derives from the linkage between the volume of precipitation and the relative humidity of the atmosphere.

 

The Thermal Differences between Land and Water

 

The continents heat up faster than the oceans, and they cool down faster too for example London has an average January low temperature of 2˚C while Winnepeg’s is closer to -20˚C, even though they are located at almost the same latitude. There are a few reasons for the land-ocean cooling differences which are given below:

 

(1)  Specific Heat Capacity. Water has a higher heat capacity than land. So it takes more heat to raise the temperature of one gram of water by one degree than it does to raise the temperature of land. 1 calorie of solar energy (any type of energy really) will warm one gram of water by 1 °C, while the same calorie would raise the temperature of a gram of granite by more than 5 °C.

 

(2)  Transparency. The heat absorbed by the ocean is spread out over a greater volume and extent. The heat can dispersed over a greater depth in water, it depends on the level of transparency.

 

(3) Evaporation. The oceans loose a lot of heat from evaporation, while there is some evaporation from wet soils and transpiration by plants, the land does not have anywhere near as much available moisture to cool it down. The land surface energy and water balances are tightly coupled by the partitioning of absorbed solar radiation into terrestrial radiation and the turbulent fluxes of sensible and latent heat, as well as the partitioning of precipitation into evaporation and runoff. Evaporation forms the critical link between these two balances.

 

(4) Currents. The oceans can observe heat energy over a greater depth and spread that energy around with their ocean currents and tides. The oceanic water moves in the form of ocean currents from equator to pole wards and from poles to equator. Currents have very important role in normalizing ocean temperature but that not happen in the case of land.

 

Soil Water Budget-Soil moisture is important for plant growth. A lack of moisture content over a growing season is a good indicator of drought, which can have social, environmental, and economic impacts. Soil moisture deficits (difference between rainfall and evapotranspiration) vary across the country and from year to year, ranging from 0 (saturated soil) to over 500mm (very dry). Soil moisture plays an important role in preventing or prolonging summer droughts. When the ground is wet, water evaporates as the day warms up. The warm, moist air rises until it encounters colder air high above the Earth’s surface, leading to afternoon rain showers. The water remains in the ground through the cool night, and the cycle repeats the next day. The Soil-water-budget is an accounting system for soil moisture using inputs of precipitation and outputs of evapotranspiration and gravitational water, it can be understand by following equation:

 

PRECIP = ACTET + SURPL ± STRGE

 

POTET – DEFIC

 

PRECIP: Precipitation (measured with a rain gauge), ACTET: Actual Evapotranspiration,

 

POTET:  Potential  Evapotranspiration,  DEFIC:  Deficit  (natural  water  shortage),  SURPL:

 

Surplus (additional water after the soil is full of moisture).

   

Evaporation is that seasonal process in which water from independent ground level evaporates and reaches in the atmosphere in the form of vapour. This process takes place when temperature reaches boiling point of thermal energy.

 

Evapo-transpiration: It is the total moisture which mixes with atmosphere as a result of evaporation and action of transpiration from plants. Rate of evapo-transpiration is mainly based on temperature. It is an important factor in climatic studies because it decides all the atmospheric conditions.

 

Potential evapo-transpiration: This process depends on the availability of water. If sufficient moisture is available to meet the needs of vegetation, the resulting evaporation is called potential evapo-transpiration.

 

Actual evapo-transpiration: is found in special conditions. Its quantity is found out by adding precipitation and changing soil moisture.

 

Infiltration:is the flow of water into the ground through the soil surface. After rain or irrigation, as soon as the water comes on the surface of the earth, it inclines to enter soil from the nearest point. Thus, entry of water into the soil is called infiltration.

 

Run-off:That part of water from rain or of melted snow which flows as drainage on the surface, is called ‘run-off.

 

Percolation: is downward movement of water in soil profile. Percolation is not the same as infiltration. It is the first stage of infiltration. Thus, till water does not percolate, its infiltration cannot take place.

 

Points to Remember

   

Porosity: the amount of pore spaces available based on soil texture and structure.

 

Permeability: is the property that determines soil recharge

 

Types of soil moisture:

 

Hygroscopic water: the portion of the soil moisture that is tightly bound to the soil particles that it is unavailable to plants.

 

Capillary water: soil moisture that is available to plant roots. It is held in the soil by the water’s surface tension and cohesive forces between water and soil.

 

Soil-Moisture Storage

 

Wilting point: the point at which the soil only has inaccessible water.

 

Field capacity: water held in the soil by hydrogen bonding against the pull of gravity, remaining after water drains from the larger pore spaces; the available water for plants.

 

Gravitational water: any water surplus after the soil becomes saturated from precipitation.

 

Zone of aeration: the area through which excess surface water moves, made of soil and rock.

 

Zone of saturation: the area where subsurface water accumulates. Basically it’s a hard sponge of rock — subsurface areas are made of both permeable and impermeable rocks. Permeable allow for the movement of water, whereas impermeable block the movement of water.

 

Water table: the upper limit of the water in the zone of saturation

 

In arid and semi-arid regions, water represents the main ecological constraint for plant survival, and hydrological processes determine the direction of evolution and ecological functioning of soil-vegetation systems. Soil moisture dynamics are the central component of the hydrological cycle (Legates et al., 2011) and are mainly determined by processes including infiltration, percolation, evaporation and root water uptake. Water movement in unsaturated soils is an inhomogeneous, nonlinear process and is influenced by climatic and environmental factors, such as precipitation, evaporation, vegetation, and soil properties. The effect of vegetation on soil water movement strongly depends on vegetation type and density (Gehrels et al., 1998). As seen in the figure 4 soil physical properties like soil texture, porosity, unsaturated flow etc. have a profound influence on soil water movement.

Fig. 4: Water Movement in different Soil Layers

 

Drought

 

Increasing temperatures and changes in rainfall patterns are expected to increase the frequency and intensity of drought in many regions.When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall, and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.The term “drought” can have different meanings to different people, depending on how a water deficiency affects them.

 

Drought is described in terms of various statistics that summarize drought duration, intensity, and severity. Droughts are generally classified into four categories i. Meteorological drought refers to a precipitation deficiency, possibly combined with increased potential evapotranspiration, extending over a large area and spanning an extensive period of time (Fig. 5 and 6). ii. Soil moisture drought is a deficit of soil moisture (mostly in the root zone), reducing the supply of moisture to vegetation. Soil moisture drought is also called agricultural drought, because it is strongly linked to crop failure.

Fig. 5: Impacts of Meteorological Drought

Fig. 6: Drought Affected Land

 

iii. Hydrological drought is a broad term related to negative anomalies in surface and subsurface water. Examples are below-normal groundwater levels or water levels in lakes, declining wetland area, and decreased river discharge.

Fig 7: Impacts of Hydrological Drought

 

4. Socioeconomic drought is associated with the impacts of the three above-mentioned types. It can refer to a failure of water resources systems to meet water demands and to ecological or health-related impacts of drought. An overview of the most important drought impacts is provided in Table 1. It can be noted that more types of drought impacts are related to hydrological drought than to meteorological drought.

Source: Irvine’s Global Integrated Drought Monitoring and Prediction System

 

Fig. 8 Distribution of Drought around the World

 

A groundwater or hydrological drought typically refers to a period of decreased groundwater levels that varies regionally and locally based on due to differences in groundwater conditions and groundwater needs for humans and the environment. Reduced groundwater levels due to drought or increased pumping during drought can result in decreased water levels and flows in lakes, streams, and other water bodies. Decreased groundwater flow to surface waters can affect aquatic ecosystems that rely on a continuous supply of groundwater to sustain aquatic habitats and stream flow. The ground water has important role in keeping water balance on the earth. The fresh water found beneath the surface which is beyond the soil-root zone is known as ground water. It is the largest potential freshwater in the hydrological cycle. Ground water level in most of the countries decreasing due to overconsumption.Ground-water systems are a possible backup source of water during periods of drought.

 

It is not unusual for a given period of water deficiency to represent a more severe drought of one type than another type. For example, a prolonged dry period during the summer may substantially lower the yield of crops due to a shortage of soil moisture in the plant root zone but have little effect on groundwater storage replenished the previous spring on the other hand, a prolonged dry period when maximum recharge normally occurs can lower ground-water levels to the point at which shallow wells go dry. If ground-water storage is large and the effects of existing ground-water development are minimal, droughts may have limited. In the absence of ground-water development and continuous Ground-water withdrawals may reduce the water flows in lakes, streams, and other water bodies can cause low water level. Likewise, reduced freshwater discharges to coastal areas during droughts may cause seawater inundation towards land beyond limits that may lead to renewed land subsidence. A common response to droughts is to drill more wells. Increased use of ground water lead to permanent, unanticipated change in the level of ground-water development. Ground-water systems tend to respond much more slowly to short-term variability in climate conditions than surface-water systems. As a result, assessments of ground-water resources and related model simulations commonly are based on average conditions, such as average annual recharge or average annual discharge to streams.

 

The effect of potential long-term changes in climate, including changes in average conditions and in climate variability, also merits consideration. Climate change could affect ground-water sustainability in several ways, including (1) changes in ground-water recharge resulting from changes in average precipitation and temperature or in the seasonal distribution of precipitation, (2) more severe and longer lasting droughts, (3) changes in evapotranspiration resulting from changes in vegetation, and (4) possible increased demands for ground water as a backup source of water supply. Climate can be a key, but underemphasized, factor in ensuring the sustainability and proper management of ground-water resources.

 

Management and conservation of water resources are critical to human welfare. The high demands for water of an increasing world population have focused our attention on water resources quality and quantity management. Climate change is likely to have significant effects on hydrological regimes, affecting both water quantity and water quality. Drought is arguably the biggest single threat from climate change. Its impacts are global. Drought triggered crisis in many middle east and African countries. Relief failures and poor drought forecasting caused several deaths in Horn of Africa during 2011 and 2012. The consequence is an increasing demand on a decreasing availability of water resources. Hydrological drought is crucial for various hydrological studies such as water quality management, determination of minimum downstream flow requirement for hydropower and ecological needs, irrigation system design and wastewater treatment.

 

you can view video on Water Balance and Drought

 

References

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