1Onset and cessation of the MarchMay rainfall season at selected stations, the length of the potential crop-growing season and the planting window in Uganda. http://sajs.co.za/index.php/SAJS/article/downloadSuppFile/470/3552
2Onset and cessation of the OctoberDecember rainfall season and the length of the potential crop-growing season at selected stations in Uganda. http://sajs.co.za/index.php/SAJS/article/downloadSuppFile/470/3553
Onset, withdrawal and length of the October–December rainfall season The results of the seasonal characteristics derived from mass curves for the selected stations where this season is dominant are presented in Table 2. The onset and cessation for the October–December season seem to be less variable within stations and more uniformly distributed at most locations compared to the March–May season (Table 2 and Figure 4). The earliest and latest onsets of rainfall at each of the stations define the planting window for that location. The number of growing (or rainy) days and the early or late onset of rainfall do not necessarily indicate a favourable or unfavourable rainfall season. In some seasons, the rains started early and withdrew early, whilst in some cases they started late but also ended late, sometimes making the durations of the seasons average or longer. Such scenarios have made seasonal forecasting challenging, with some stakeholders suggesting that it is necessary to link forecasting with indigenous knowledge to make it more precise and relevant to the farmers. Intraseasonal variations In the bimodal rainfall regime represented by Namulonge Station in central Uganda, there seems to be a decreasing trend in the number of rainy days during the months of April and May and in the amount of rainfall during the month of April (Figures 5 and 6); unfortunately, April and May are the critical months of crop growth. The decrease in the amount of rainfall and the number of rainy days is manifested in unseasonable periods of no rain lasting from about 3 to 4 weeks interspersed within the rainy season. These unseasonable periods of no rain are becoming a common occurrence during the March–May season. This phenomenon renders crops grown in this season prone to climatic risks and therefore in need of adaptation measures. In their study, Jennings and Magrath6 also noted that, within recognisable seasons, unusual and unseasonable events are occurring more frequently, for example, heavy rains in dry seasons, dry spells in rainy seasons and storms at unusual times. For Uganda in particular, they reported that farmers have noted increasingly unreliable rainfall during the March–May season, that is, the rain does not fall consistently throughout the season but rather comes in short, often localised, torrents interspersed with hot, dry spells. 6The relationship between the number of rainy days and the years, as well as between the amount of rainfall and the years, in the months of April and May were established through linear regression. Although the r2-values are very small (Figures 5 and 6), the response of crop yield is non-linear and, in most cases, exponential. Therefore small spikes of moisture stress at the critical biological processes of yield formation can lead to a reduction in crop yields. The trends reflected on the graphs for April and May might be very significant in this regard, depicting a trend of concern during the first growing season in the areas represented by Namulonge Station. In the transition zone represented by Soroti Station in eastern Uganda, the amount of rainfall during the months of November and January exhibited an increasing trend, whilst rainfall during the peak month of May showed a slight decreasing trend (Figure 7). North-eastern Uganda, represented by Kotido Station which is generally dry, experienced a unimodal rainfall regime commencing from April to November, with peak rainfall during April, May, July and August and a decrease during the month of June (Figure 8). This pattern has been consistent for years, as reported by Wilson 30and Musiitwa and Komutunga 31, who observed that the rainfall in the subregion is characteristically episodic in occurrence, alternating with a prolonged severe dry season. They further noted that there is considerable variation from year to year in the total annual rainfall and moreover that the rainfall is not well distributed. 30,31 This observation is well illustrated in Figures 8 and 9: monthly and annual rainfall variability is high but without a consistent pattern. Further analysis shows that average monthly rainfall in June has been steadily increasing, whilst average monthly rainfall in September and October shows a declining trend (Figure 8). Although the observed increase in the monthly rainfall in June is beneficial, as it comes in the middle of the season, the observed decrease in rainfall at the end of the season is detrimental, as it shortens the already short annual length of the potential crop-growing period of the region, posing challenges to pasture and crop-growing. Annual rainfall also shows a decreasing trend (Figure 9). Quantitatively, the rains received annually have decreased by about 15% – 20% since the 1960s. According to Anderson and Robinson 19, average annual rainfall has decreased by about 15%, but the deficit is further compounded by the way the rainfall arrives, because the intensity and duration between rainfall events has varied considerably. No longer can periods of reliable rainfall be assumed in one year out of every three. The trends in rainfall across Uganda, described above, are a result of the complex interactions between the diverse topographical features of the country, the lakes and rivers and associated expanses of swamps, the wind systems over the country (including the trade winds), the intertropical convergence zone and the pressure systems. The rainfall regimes of Uganda have also been found to have teleconnections with sea surface temperatures in the Pacific, Indian and Atlantic Oceans, and the ENSO phenomenon. 26Ogallo 26further noted that the behaviour of these global systems resulting from global warming in turn affects the regional pressure systems, which in turn affect the local systems, resulting in the observed trends in rainfall over the country. The persistence of warm conditions over the Indian Ocean near Madagascar sucks in air from the East African region, depriving the region of moisture and creating dry conditions within the region. The western parts of the region, including Uganda, benefit from the moist air pulled in from the Congo forest and receive a rainfall boost. During the warm phase of the Pacific Ocean (El Niño) the region receives sufficient moisture, whilst in the cold phase (La Niña) the region receives little rainfall. Temperature trends Figure 10 shows the average daily maximum temperatures from 1950 to 2008. The temperature trend clearly shows that there has been an increase in temperature during this period. However, the lower limit of the range of daily maximum temperatures showed a faster rate of increase than the upper range. According to Mubiru et al. 3, the lower limit of the range of daily minimum temperatures also increased faster than the upper limit. The implication of these observations is that the day and night temperatures are becoming warmer. 3As Thornton et al. 13noted, the impacts of an increase in temperature, however small, will be far-reaching. ConclusionsWe have generated information on seasonal rainfall characteristics relevant in exploiting the agricultural possibilities offered by climatic variability. Frequently, the onset of rains in the March–May season is delayed for as many as 30 days, with rains starting in mid-April instead of mid-March. However, the timing of rainfall cessation has more or less stayed the same, regardless of the time of onset of rainfall. Consequently, even when rains start late, withdrawal is timely, thus making the growing season shorter. In contrast, onset and cessation of the October–December season are less variable within stations and more uniformly distributed at most locations. At stations experiencing a unimodal rainfall regime, the average onset of rains is also quite stable. On a monthly scale, there seems to be a decreasing trend in the number of rainy days during the critical months of crop growth in the March–May season, making crops grown in this season prone to climatic risks and therefore in need of adaptation measures. The average daily maximum and minimum temperature trends reveal an increase in temperature over the 50-year period. However, the lower limits of the ranges of the daily maximum and minimum temperatures are increasing faster than the upper limits. The implication of this finding is that the day and night temperatures are becoming warmer. The seasonal information thus generated offers opportunities to exploit the seasonal distribution of rainfall to improve and stabilise crop yields through the incorporation of the seasonal characteristics of the onset, cessation and length of the crop-growing season. This information can also guide crop substitution and diversification. AcknowledgementsWe are grateful to the Meteorological Department, Kampala, Uganda. The research was supported by the National Agricultural Research Organization, the National Agricultural Research Laboratories-Kawanda, and UNEP Risø Centre and funded by UNDP/UNEP under the terms of Grant No. 1215186-03 Subcontract-Uganda NARL/ NARO project. Competing interests We declare that we have no financial or personal relationships which may have inappropriately influenced us in writing this article. 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