diagnosis and monitoring of drought

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Diagnosis and Monitoring Of Drought Using Regional Climate Model Over BangladeshIntroduction1.1 PreambleDrought is a prolonged, continuous period of dry weather along with abnormal insufficient rainfall. It occurs when evaporation and transpiration exceed the amount of precipitation for a reasonable period. Drought causes the earth to parch and a considerable hydrologic (water) imbalance resulting water shortages, Wells to dry, depletion of groundwater and soil moisture, stream flow reduction, crops to wither leading to crop failure and scarcity in fodder for livestock. Drought is a major natural hazard faced by communities directly dependent on rainfall for drinking water, crop production, and rearing of animals. Since ancient times droughts have far-reaching effects on mankind. Large land areas often suffer damages from dust storms and fire. Drought could be the reason for migration of early human communities. It has long been considered to be a natural hazard responsible for us and downs of many civilisations of the world. It is the slow onset natural disaster, which commences before anybody notices it and by the time it is noticeable it is too late. Unlike other natural disasters, it starts unnoticed and develops cumulatively, thus its impact is not immediately observable by naked eye or ground data. It may be the most devastating, yet least understood of all weather phenomenon. Drought can erupt in a matter of months, or it can gradually creep up on an unsuspecting society over several seasons. Drought is rarely forecasted with any skill, and goes unobserved by the public until Impacts from the drought have already occurred. Inevitably, officials charged with mitigating those impacts want to know how a current drought measures up historically to other droughts in terms of intensity, areal coverage, and duration. Additionally, these three factors differ in relative time and space scales from drought to drought.Meteorological drought is a short lived, recurring natural disaster, which originate from the lack of precipitation and can bring significant economic losses [Pal et al., 2000]. It is not possible to avoid meteorological droughts, but it can be monitored, and their adverse impacts can be alleviated [Gommes, 1994]. The success of the drought prediction depends on how well it is defined and identified.Bangladesh is one of the most seriously affcteded countries suffering from meteorological disasters such as droughts in pre- and post-monsoon seasons and floods in the summer monsoon, tropical cyclones and meso-scale disturbances. Additionally, agriculture, power generation and industrial production subtantially depend upon precipitation (Devkota, 2006). In view of the critical influnence of large inter-annual variability of precipitation on agricultural and industrial production, seasonal prediction of drought becomes very important for policy making efforts [Giorgi et al., 1996]. Beside the other natural disasters, Bangladesh face drought situation. Now-a-days food security is an important issue in the World. Because drought is intimately related with food security, therefore, its diagnosis and monitoring is essential to carry out too. The diagnosis of drought is also important for the utilization of drought projection using climate modeling facilities for the stakeholders and planners of a country.1.2 Drought: DefinitionsDroughts have no universal definition. As drought definitions are region specific, reflecting differences in climate characteristics as well as incorporating different physical, biological and socio-economic variables, it is usually difficult to transfer definitions derived for one region to another. However some of the common definitions of drought can be noted as under:

The Director of Common Wealth Bureau of Meteorology in 1965 suggested a broad definition of drought as severe water shortage.

Definition given by Palmer states that Drought is an interval of time, generally of the order of months of years in duration, during which the actual moisture supply at a given place rather consistently falls short of the climatically expected or climatically appropriate moisture supply (Palmer, 1965).

According to Mc Mohan and Diaz Arena (1982), Drought is a period of abnormally dry weather sufficiently for the lack of precipitation to cause a serious hydrological imbalance and carries connotations of a moisture deficiency with respect to mans usage of water.

Another definition given by Flag is worth mentioning Drought is a period of rainfall deficiency, extending over months to year of such a nature that crops and pasturage for stock are seriously affected, if not completely burnt up and destroyed, water supplies are seriously depleted or dried up and sheep and cattle perish.

According to Hangman (1984), Drought is considered by many to be the most complex but least understood of all natural hazards affecting more people than any other hazard.

According to National Drought Policy Commission, A persistent and abnormal moisture deficiency having adverse impacts on vegetation, animals, and people. According to The Convention to Combat Desertification (CCD), drought means the naturally occurring phenomenon that exists when precipitation has been significantly below normal recorded levels, causing serious hydrological imbalances that adversely affect land resources production systems.A drought is a complex phenomenon that can be defined from several perspectives. Wilhite and Glantz ((2000) categorize drought definitions into conceptual (definitions formulated in general terms) and operational. Conceptual definitions formulated in general terms; help people understand the concept of drought but these normally do not provide quantitative answers. Operational definitions on the other hand help identify the drought beginning, end and degree of severity.

By studying the above definitions it can be understood that drought is mainly concerned with the shortage of water which in turn affects availability of food and fodder thereby leading to displacement and loss to economies as a whole. 1.3 Classification of Drought Drought can be classified in three main ways:

Meteorological drought: related to rainfall amounts

Hydrological drought: determined by water levels in reservoir

Agricultural drought: related to the availability of water for crops.

1.3.1 Meteorological DroughtMeteorological drought is generally defined by comparing the rainfall in a particular place and at a particular time with the average rainfall for that place. The definition is, therefore, specific to a particular location. Meteorological drought leads to a depletion of soil moisture and this almost always has an impact on crop production. When we define drought this way, we only consider the reduction in rainfall amounts and don't take into account the effects of the lack of water on water resevoirs, human needs or on agriculture.

Meteorologically drought can be classified into three types: permanent drought - characterised by arid climate; seasonal drought - caused by irregularities in recognised rainy and dry seasons; and contingent drought - caused by irregular rainfall. In Bangladesh, the last two types are more prevalent.

1.3.2 Hydrological Drought

Hydrological drought is associated with the effect of low rainfall on water levels in rivers, reservoirs, lakes and aquifers. Hydrological droughts usually are noticed some time after meteorological droughts. First precipitation decreases and, some time after that, water levels in rivers and lakes drops.

Hydrological drought affects uses which depend on the water levels. Changes in water levels affect ecosystems, hydro electrical power production and recreational, industrial and urban water use. 1.3.3 Agricultural drought

Agricultural drought occurs when there is not enough water available for a particular crop to grow at a particular time. This drought doesnt depend only in the amount of rainfall, but also on the correct use of that water. Imagine a period of low rainfall where water is used carelessly for irrigation and other purposes. Under these circumstances, the effect of the drought becomes more pronounced than it was before.

Agricultural drought is typically seen after meteorological drought (when rainfall decreases) but before a hydrological drought (when the water level in rivers, lakes and reservoirs decreases).It is important to mention that the effects of droughts are different in irrigated and non-irrigated agriculture. In regions which rely on irrigation, the impacts of short lived agricultural droughts are usually lower than in regions where crops are not irrigated. Irrigated agriculture relies on stocks of water so if it doesn't rain, these crops still get the water they need (until the reservoirs run dry). However, in non-irrigated agriculture crops depend directly on the rain as their water source. If it doesn't rain, the crops don't get the water they need to survive.

1.4 Drought in BangladeshBangladesh extends from 20o34'N to 26o38'N latitude and from 88001'E to 92041'E longitude. Climatically, the country belongs to sub-tropical region where monsoon weather prevails throughout the year. The average temperature of the country ranges from 17 to 20.6oC during winter and 26.9 to 3l.1oC during summer. Four distinct seasons can be recognized in Bangladesh from climatic point of view: (i) the winter season from December to February, (ii) the pre-monsoon season from March to May, (iii) the monsoon season which lasts from June to September and (iv) Post-Monsoon season lasting from October and November. The average annual rainfall of the country varies from 1329 mm in the northwest to 4338 mm in the northeast (Shahid et a1., 2005). The g

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