2.3.2. Rainfall and Scenario Analyses

Rainfall data during the years 1998–2014 were obtained for each scheme from the Tropical Rainfall Measuring Mission (TRMM) database. The total annual rainfall and total number and duration of dry spells were determined by the following conditions: (i) onset of rainfall is the beginning of a 10 day period between the second dekad of April and the first dekad of May during which the cumulative rainfall is ≥25 mm, and a dry spell ensuing within 30 days from the start of the 10 day period is ≤8 days [1,42]; (ii) cessation of rainfall is the last rainfall event between the third dekad of September and the second dekad of October [42]; (iii) dry spell is two or more consecutive non-rainy days [7], as even a period of two days without rainfall at critical growth stages is detrimental to crop production in savannah environments, particularly during periods of low rainfall; (iv) frequency of dry spells is the number of dry spells during the rainy season in the particular year under focus.

Additionally, the inter- and intra-seasonal variability of rainfall was expressed in the coefficient of variation based on the annual and monthly rainfall data, respectively:

$$CV = \frac{\sigma}{\mu} \ast 100 \,\mathrm{[\,\%]}\tag{3}$$

where *CV* is the coefficient of variation, *σ* is the standard deviation, and *μ* the mean of the rainfall data.

For the estimation of the supplemental irrigation requirement for maize, two climate scenarios (i.e., wet and dry rainfall regimes) were formulated based on rainfall amount and the frequency of dry spells. The first scenario (S1) was a wet year characterized by ≤20% probability of exceedance (i.e., the likelihood of the occurrence of rainfall ≥1057 mm) and by less frequent dry spells [43]. The second scenario (S2) was a dry year characterized by ≥80% probability of rainfall occurrence exceeding 796 mm and by frequent dry spells [43] (Figure 4).

**Figure 4.** Probability plot of the total annual rainfall for the Vea and Bongo irrigation schemes for 1998–2014.
