In the sudan savanna of northern Nigeria, with its semi-arid climate, the ability to determine effectively or predict the start of actual productive rains cannot be overemphasized. Several methods exist for calculating the date of onset of the rains that may be taken as the start of the growing season. Five methods currently in use, which are relatively easy to apply on a large scale, were selected for comparison. One is a traditional technique (Ramadan method), two use accumulated rainfall totals (Walter's and Sivakumar's methods), and two use rainfall-evapotranspiration relationships (Kowal's and Benoit's methods). For the period 1961-91, the traditional technique performed most poorly. Walter's method gave quite early onsets and Sivakumar's method gave very late onsets, thereby seriously shortening the growing season. Kowal's and Benoit's determinations fell most often in between the results of Walter's and Sivakumar's methods in their performance. However, although generally to a lesser extent than the other methods, they are still significantly affected by false starts. To avoid incorrectly predicting the growing season's onset as far as possible, but to prevent an unacceptable shortening of the growing season, a combination of Kowal's and Sivakumar's criteria was used to develop an improved technique. This proved to work well for determining the onset date in the study area. However, because some false starts remain when using average onset dates, it is suggested that an operational advisory team should be constituted by the government. This team would be responsible for calculating onset dates in any year, on-line for the ongoing season, in a participatory approach with farmers, and for disseminating such dates to the farmers. This could be done for any place for which the appropriate data can be made available. In the future, improved climate prediction skill may replace the classical probabilistic approaches presently suffering from increasing rainfall variabilities.

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Mean growing season (April-September) precipitation series at 407 stations in the Interior Plains of North America, dating back to 1985 in some stations, are used to derive a Bhalme and Mooley-type drought index for the region. Linear trend, representing first order non-stationarity, is first removed from the drought series. Then the periodogram via the Fast Fourier Transform (FFT), Maximum Entropy (ME) and Non-Integer (NI) spectral techniques are applied to search for significant periodicities in the temporal characteristics of the drought series. Cross-spectral technique is also used to characterize the extent of spectral coherence in the drought series.It is found that no significant power in drought is present at periods greater than about 6 years. Spectra of drought in the Interior Plains are dominated by three peaks in the quasi-biennial, quasi-triennial and quasi-5-year oscillations. The periodic 18.6-year lunar nodal cycle obtained by others for the Drought Area Index (DAI) for the region is completely absent in the spectra of the analysed drought index series. Also the well-known 11-year sunspot cycle is only evident in the drought index series for a few of the stations. In general there is an ill-defined drought recurrence interval.Moreover the areal extent of peaks contained within these oscillations do not indicate any significant spatial coherence. These periodicities can be evident at one station while being absent at another a few hundred kilometres away. On the basis of the spectral results, it is concluded that obtained significant periodicities are only short-lived time variabilities in drought as recorded by individual stations and do not represent any persistent drought characteristics over a broad geographical region. A cross-spectral analysis of the drought series with sunspot numbers shows no evidence to indicate a significant relation between drought and the single sunspot cycle. In general, the growing season drought series in the Interior Plains displays only what appears to be a random variation, with the possible superimposition of the quasi-biennial oscillation.KEY WORDS Drought Periodogram maximum entropy and non-integer spectral techniques Drought spectral coherence Cycles

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Nigeria is Africa's largest economy and home to approximately 10% of the unelectrified population of Sub-Saharan Africa. In 2017, 77 million Nigerians or 40% of the population had no access to affordable, reliable and sustainable electricity. In practice, diesel-and petrol-fuelled back-up generators supply the vast majority of electricity in the country. In Nigeria's nationally-determined contribution (NDC) under the Paris Agreement, over 60% of the greenhouse gas emissions (GHG) reductions are foreseen in the power sector. The goal of this study is to identify and critically examine the pathways available to Nigeria to meet its 2030 electricity access, renewables and decarbonization goals in the power sector. Using published data and stakeholder interviews, we build three potential scenarios for electrification and growth in demand, generation and transmission capacity. The demand assumptions incorporate existing knowledge on pathways for electrification via grid extension, mini-grids and solar home systems (SHS). The supply assumptions are built upon an evaluation of the investment pipeline for generation and transmission capacity, and possible scale-up rates up to 2030. The results reveal that, in the most ambitious Green Transition scenario, Nigeria meets its electricity access goals, whereby those connected to the grid achieve a Tier 3 level of access, and those served by sustainable off-grid solutions (mini-grids and SHS) achieve Tier 2. Decarbonization pledges would be surpassed in all three scenarios but renewable energy goals would only be partly met. Fossil fuel-based back-up generation continues to play a substantial role in all scenarios. The implications and critical uncertainties of these findings are extensively discussed.Key policy insights . The 2030 electricity mix for Nigeria varies across the scenarios presented, with the most ambitious scenario achieving electricity access goals and partly meeting renewable energy goals. . All three scenarios surpass the decarbonization targets of Nigeria's NDC for the power sector. . The transformation of the power sector relies on a wide range of financial, policy and enabling environment-related conditions taking place in the near-term, some of which are in turn strongly influenced by larger political economy realities. . Fossil fuel-based back-up generation plays a substantial role in all scenarios. Data availability for this technology remains a significant source of uncertainty. ARTICLE HISTORY