Rainfall is one of the key climatic variables [1
], and its temporal variability over the Lake Victoria Basin (LVB) has far reaching effects on the social, economic and ecological aspects of the region [4
]. The LVB has an area of about 198,000 km
and is home to about 340 million people [6
]; most of whom depend on rain-fed agriculture and fishing [7
]. Previous studies on areal rainfall over the LVB found a strong influence of rainfall on agriculture and hydrology. For example, Sabiiti et al. [8
] found that banana yields in the LVB could reduce by 46% due to reduced rainfall. Over the LVB, heavy rainfall has normally caused floods, which are exacerbated by poor drainage systems, especially in urban areas [9
]. Rainfall is also the major source of water for Lake Victoria, which is the main source of the Nile River [4
The rainfall over the LVB is affected by a number of factors, including the Inter-Tropical Convergence Zone (ITCZ), El Niño/La Niña episodes, the Indian Ocean Dipole, as well as quasi-biennial oscillation and other extra-tropical weather systems [4
]. The diurnal variations are influenced by lake-land circulation [4
], and these have an influence on a number of activities, like fishing and tourism. Anyah et al. [5
] also attribute the inter-annual variability of rainfall over LVB to the periodic episodes of anomalously wet or dry conditions associated with sea surface temperature anomalies over the equatorial Indian Ocean and also to the Pacific Ocean sea surface temperature perturbations.
The rainfall variability on different temporal scales, such as the inter-annual, seasonal and inter-decadal, is extensively studied globally as observed by Barron et al. [11
]. For example, Goswami et al. [12
] studied rainfall variability over central India and found rising trends in the magnitude and frequency of extreme rain events during the monsoon from 1951–2000. Cheung et al. [13
] found an increasing trend of June–September rainfall over Ethiopia. Kizza et al. [4
], and Nsubuga et al. [2
] found a positive rainfall trend over the 20th century, while Awange et al. [6
] found a slight increasing annual rainfall trend over LVB. Additional seasonal rainfall investigation has been carried out by Nimusiima et al. [14
] who found the potential of the December–February season becoming wetter over Uganda for the period 2020–2050, and Awange et al. [6
] expect these projected changes in rainfall over LVB to impact the population around LVB.
The intra-seasonal rainfall studies have been carried out over different areas for different seasons and using different methods. For example, Bowden and Semazzi [15
] used empirical orthogonal analysis for the October–December season from 1979 to 2001. They observed that the dominant cause of October–December seasonal rainfall variations was the El Niño/La Niña episodes and Indian Ocean Dipole. However, they did not consider intra-season rainfall characteristics, which are important especially in agriculture production. A thorough analysis of intra-seasonal rainfall variability should attempt to answer the questions: (1) What period of season has a rainfall reduction/increase? (2) What are the characteristics of the onset and cessation of seasonal rainfall? Grouping seasonal rainfall into dekads can segment the periods of the season. A dekad is a ten-day rainfall period. We studied the variation of seasonal rainfall within dekads to illustrate the period within the March–April–May (MAM) season that have an increase/reduction in rainfall.
Over the LVB, many studies about rainfall variability have been carried out, such as Awange et al. [6
], Kizza et al. [4
] and Anyah et al. [5
]. Kizza et al. [4
] suggested the need of constantly carrying out temporal analysis of precipitation using updated datasets in order to analyze the current trends in precipitation over LVB, and Hartter et al. [16
] urged the need of using fine-scale climatic information on trends. Additionally, Ogwang et al. [1
] illustrated the necessity of having a clear understanding of the past climatic trends. Studies by Nimusiima et al. [17
] established that the community believed there was a changing temporal and spatial rainfall pattern and that seasonal rainfall has become unpredictable. What is not clear are the intra-seasonal changes regarding wet and dry spells. The community perceives that dry spells have become longer, but is not sure which period (month) of the season is more affected than the other.
Our study addresses the uncertainty regarding intra-seasonal rainfall variability with the focus on dekadal trends of the MAM season. We aimed at identifying the periods within the MAM season that are becoming drier or wetter and considered variability in rainfall characteristics over the LVB because this variability is important due to its impact on water levels of the lake [4
], health [18
] and agriculture [8
]. The objectives of our paper were: (1) to examine the dekadal rainfall trends during the MAM season; (2) to analyze the trend of dekadal rain days during the MAM season; and (3) to assess the trend of rainfall events (i.e., light rain days and wet days).
4. Summary and Conclusions
The main aim of our study was to investigate the intra-seasonal rainfall variability over LVB by analyzing dekadal rainfall. The daily rainfall obtained from UNMA was first treated to the normality test using Shapiro–Wilk’s normality test and homogeneity test using double-mass curves. Sixty five percent of the data was found normally distributed at the 95% confidence level, and all stations had homogeneous rainfall data records. The missing rainfall data were less than 5% and filled using the modified normal ratio method. The rainfall data were then classified in dekads from which dekadal rainfall amount and dekadal rain days were computed. The average rainfall and rain days during the March–May and September–November seasons over LVB was almost similar in magnitude, and we also noted that the March–May season had fewer light rain days and more wet days than the September–November season.
The trend of accumulated rainfall and the total number of rain days over a given dekad were analyzed using the Mann–Kendall trend test and simple linear regression and presented using tables and figures. The majority of the trends were generally not significant at the 95% confidence level, but consistent and pointed to an increasing trend in both accumulated rainfall and total number of rain days during the third and seventh dekads and decreasing trends during the 1st, 2nd and 6th dekads. We also noted a greater and significant increase of dekadal rainfall of the seventh dekad compared to the third dekad over the last ten years (2006–2015) and a corresponding significant decrease in dekadal rainfall during the eight dekad.
Additionally, we found that the trend of light rain days was generally twice as much as the increase of wet days, which was also evident during the third and seventh dekads. We also analyzed the trends of dekadal rainfall intensity and found them consistent with earlier results showing increasing trends during the third and seventh dekads and decreasing trends for the first and sixth dekads. Analysis of consecutive dry days showed that 2–4 days with consecutively no rainfall accounted for 74%, and we did not find a significant trend. The days with extreme rainfall increased significantly during the third and seventh dekads. Since other studies over the same region have pointed to a ‘no significant change’ in March–May seasonal rainfall regarding on-set and cessation, our results suggest intra-seasonal shift in March–May seasonal rainfall, making the third and seventh dekads become wetter.