Agriculture is primarily and heavily dependent on climate. The climatic factors-rainfall, sunshine hours, temperature, relative humidity and length of the drought period-result in cycle-to-cycle variability of crops production. The uncontrollable natures of climate factors are changing over time affecting agricultural, economic, social and environmental sustainability of a country.
Different trends show an increase in average temperature and more volatile rainfall patterns (National Research Council, 2001). Rainfall is one of the most important climatic variables because of its two sided effects - as a deficient resource, such as droughts and as a catastrophic agent, such as floods. Several studies have been carried out on rainfall at different temporal scales - from daily to annual and in different areas. It is expected to result in long-term water and other resource shortages, degrading soil condition, disease and pest outbreaks on crops and livestock and so on. It affects different crops differently. Therefore, changes in outputs and economic returns from different crops differ significantly which in turn also affects the corresponding crop growers differently. Farmers will be expecting losses, primarily, due to reductions in agricultural productivity, crop yields and loss of farm productivity.
About 60% of the world and 90% for sub-Saharan African staple food production are under direct rainfed agriculture (Savenije, 2001). However, in sub-Saharan Africa most of the crop failures are due to deficit in soil moisture (Hatibu et al., 2000) caused by dry spells. Previous studies have found that annual decreases in precipitation in eastern part (Amanatidis et al., 1993; Kutiel et al., 1996), central part (Piervitali et al., 1998) and western part (Esteban-Parra et al., 1998; De Luis et al., 2000) of Mediterranean areas. While working on the island of Sicily (Italy-Southern centre of Mediterranean Sea), found a general decreases of annual rainfall with a decreasing trend for some rain gauges around Palermo area (Aronica et al., 2002). Bonaccorso et al. (2005) analyzed the trends of annual maximum rainfall series of Mediterranean areas and found different behavior pattern based on the different time scale, particularly shorter duration series show increasing trends and longer duration series show decreasing trends.
On the other hand, averaged precipitation is expected to increase globally (Houghton et al., 2001) while the magnitude of regional precipitation changes varies among models, with the range 0-50% where the direction of change is strongly indicated and around -30% to +30% where it is not. Projection shows increase in northern high-latitude regions in winter, whereas reduces in subtropical latitudes (Giorgi and Francisco, 2000). For some areas, it shows a positive trend in the daily intensity and a tendency toward higher frequencies of extreme rainfall in the last few decades (Intergovernmental Panel on Climate Change, 1995). Among them, the main areas where significant positive trends have been observed are USA (Karl et al., 1995; Trenberth, 1998), eastern and north-eastern Australia (Suppiah and Hennessey, 1998; Plummer et al., 1999), South Africa (Mason et al., 1999; Hamdi et al., 2009), UK (Osborn et al., 2000) and northern and central Italy (Brunetti et al., 2000; 2001; Aung et al., 2009; Kafaki et al., 2009).
Fuhrer et al. (2006) reviewed on Europe that both rain-day frequency and intensity during winter increases towards the north (about 45[degrees]N), while the rain-day frequency decreases to the south. This is also consistent with the increases of mean winter precipitation by 10-30% over most of central and northern Europe and decreases over the Mediterranean. In the summer, the most notable change is strong decreases in the frequency of wet days; for instance, about half in the Mediterranean, along with a 20-50% decrease of mean summer precipitation. In the tropics, models show an increase in Africa, a small increase in South America, but no change in Southeast Asia. Summer precipitation is expected to decrease in the Mediterranean-basin and in regions of Central America and north-western Europe. Most cases when there is a positive trend in rainfall intensity, an increase in total precipitation has also been observed (Groisman et al., 1999). However this relationship is not universal. Observation shows there is an increase in heavy precipitation in some areas (i.e., Italy) with a tendency toward a decrease in total precipitation (Brunetti et al., 2001).
Agriculture is the principal user of all water resources, such as, rainfall (so-called green water) and water in rivers, lakes and aquifers (so-called blue water). Irrigation is responsible for about 72% of global and 90% of developing country water withdrawals (FAO, 2003). Around 80% of the world's agricultural land is rainfed which contributes at least two-thirds of global food production. At the same time, irrigation plays an important role in supplying food. About 20% cropland of the world is irrigated, with a major fraction located in Asia, producing about 40% of the global crop yield annually (Newton, 2007). In spite of the higher risks in rainfed agriculture, especially in drought-prone areas, there is no alternative but most of the food comes from rainfed agriculture.
On the other hand, rainfall plays a destructive role for agriculture. Heavy rainfall at the end of the crop cycle causes damages of crops and financial losses to the farmers. Excessive rainfall also causes flood that makes huge damages of crops and farmers' effort useless.
In respect to Malaysia, the simulated results indicate both increases and decreases in rainfall that causes serious concern for agricultural production. The changes in rainfall may fluctuate from about -30% to +30%. This change will reduce crop yield and many areas will be prone to drought hence becoming unsuitable for the cultivation of some crops such as rubber, oil palm and cocoa. Rainfall variability increases the level of environmental stress that affects the capability of the system to maintain productivity (Tisdell, 1996). The actual farm yields of rice in Malaysia vary from 3-5 tons per hectare, where potential yield is 7.2 tons.
Total yearly rainfall in Malaysia is increasing but its monthly variation is too high. Higher rainfall in a certain time affects the agricultural outcome more destructively. Though the effect of lower rainfall is reasonably possible to check through proper irrigation system, the opposite phenomenon of over rainfall in any particular time, especially at the end of the crop cycle or at the maturating period of crop, causes serious damages to crops, which is absolutely...