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We find no evidence that the delayed onset of the wet season over Nigeria during April - May 2015 was made more likely by anthropogenic influences or anomalous sea surface temperatures.

In West Africa, which is an extremely moisture-limited region, soil water information plays a vital role in hydrologic and meteorologic modeling for improved water resource planning and food security. Recent and upcoming satellite missions, such as SMOS and MetOp, hold promise for the regional observation of soil moisture. The resolution of the satellites is relatively coarse (> 100 [km.sup.2]), which brings with it the need for large-scale soil moisture information for calibration and validation purposes. We put forward a soil moisture sampling protocol based on hydrotopes. Hydrotopes are defined as landscape units that show internally consistent hydrologic behavior. This hydrotope analysis helps in the following ways: 1) by ensuring statistically reliable validation via the reduction of the overall pixel variance and 2) by improving sampling schemes for ground truthing by reducing the chance of sampling bias. As a sample application, we present data from three locations with different moisture regimes within the Volta Basin during both dry and wet periods. Results show that different levels of reduction in the overall pixel variance of soil moisture are obtained, depending on the general moisture status. With respect to the distinction between the different hydrotope units, it is shown that under intermediate moisture conditions, the distinction between the different hydrotope units is highest, whereas extremely dry or wet conditions tend to have a homogenizing effect on the spatial soil moisture distribution. This paper confirms that well-defined hydrotope units yield an improvement at pixel-scale soil moisture averages that can easily be applied. Index Terms--Geostatistics, hydrotopes, MetOP, satellites, scaling, soil moisture, Soil Moisture and Ocean Salinity (SMOS), West Africa.

ABSTRACT Information on trends and changes in moisture conditions over a region can help policymakers to develop strategies for water resources and disaster risk management. In this study, the spatiotemporal variation of moisture indices and their annual trends in Nigeria were investigated. Thornthwaite moisture index ( MI), the standardized precipitation index ( SPI) and the standardized precipitation-evapotranspiration index ( SPEI) were computed for 1951-2014 period. Mann-Kendall trend and Sen's slope tests were used to examine the existence of temporal and spatial changes in these indices. MI decreased with increasing latitude and values ranged from −86 in the extreme north to +62 in the eastern part of the Niger Delta. A total of 73.4% of the landscape is under conditions where water supply is generally below the evaporative demand and only 26.6% of the area falls within wet sub-humid and humid regimes. Significant downward trends were observed for all the moisture indices in most of the zones. In general, negative trends were observed in 98.2% of the landscape for MI, 96.7% for SPI and 98.2% for SPEI, showing drying tendencies over Nigeria. Between 40 and 50% of the land area is manifesting persistent shift towards aridity. Observed trends in MI is strongly correlated with that of SPI ( r = 0.805) and SPEI ( r = 0.715) and correlation of 0.886 was found between changes in SPI and SPEI. Rainfall generally showed higher and significant correlations with MI and slopes of all moisture indices. This information could help policymakers to develop appropriate coping and mitigation strategies for the most vulnerable areas. [ABSTRACT FROM AUTHOR]

Adequate water resources management at the basin level needs quality downscaling of climate change scenarios for application to impact assessment and adaptation work. This study evaluates the ability of a regional climate model (RegCM3) to simulate the present-day climate and regional water balance over the Niger River Basin (NRB). RegCM3 gives a good simulation of the NRB hydroclimatic features. The mean bias error for monthly temperature is 1.5°C, 0.3 mm d-1for rainfall, and 0.4 mm d-1for runoff. Moderate to high correlations (0.66–0.95) were found between the modelled and the observed variables. RegCM3-based water cycling indices were not statistically different from the observation. Seasonal moistening efficiency (m) ranges between 19% and 37%; 66% of the available atmospheric moisture over NRB precipitates between June and September, of which 21% originates from local evaporation. The result suggests that the moisture sink period is July to October with very high precipitation efficiency over the basin. The model reproduces the hydroclimatology of the NRB and hence is a suitable tool for further studies relating to the assessment of climate change impacts on river basin water systems.Editor Z. W. Kundzewicz; Associate editor D. Hughes [ABSTRACT FROM AUTHOR]

Understanding changes in evaporation rates is expected to be of great importance for water resource planning and management. This paper examines pan evaporation data as an example of the detection and attribution of trends in climate variables. Records of pan evaporation ( Epan), rainfall ( R), radiation ( SR), wind speed ( WS), temperature ( Ta) and humidity ( RH) for the period 1973-2008 were collected from the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria. Mann-Kendall trend and Sen's slope tests were performed on the respective meteorological variables and a variability index (VI) was also computed for these. The results showed that annually Epan, SR and WS significantly decrease ( P < 0.001) while R, Ta, and RH showed insignificant increasing trends in the last four decades. Epan and SR decrease at the rate of 8.3 mm year−2 and 37.8 MJ m−2 year−1, respectively. Similar to Epan, SR decreased significantly in all the months ( P < 0.01) and the reduction ranged from 5.1% per decade in March to 9.3% per decade in August. The result of VI showed that the decrease in Epan and other explanatory variables is rather recent. Regression between Epan and other variables indicates that about 30, 15 and 6% of its variance can be explained by SR, WS and VPD (vapour pressure deficit), respectively. The possible roles of dust-haze known as 'harmattan' winds and monsoon clouds in attenuating SR and hence reducing Epan are discussed. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Rapid and reliable estimates of leaf area index (LAI) are important for studies of exchanges of energy and gases in the biosphere-atmosphere continuum. This paper evaluates the field performance of SunScan canopy analysis system for rapid estimation of LAI. Direct and indirect measurements of LAI were made in a maize (Zea mays L.) field at four phenological stages (emergence, vegetative, flowering and physiological maturity) at a tropical site in Ghana during the Glowa Vota Project field campaign (www.glowa-volta.de). Similar measurements were repeated in early and late planting seasons with similar crop management practices. The result showed a generally good performance of this sensor at all the phenological stages. Average LAI from the sensor (LAIS), ranged from 0.40-4.45, and was consistently higher than the actual LAI, which varied from 0.31-4.22, respectively for both seasons. Regression between LAI and LAIS showed a range of significant correlations with R² > 0.74 for all the stages and seasons. With combined datasets for all stages and the two plantings, a simple regression model was fitted to estimate LAI from LAIS with R² = 0.97 and standard error of 0.23 (P < 0.0001). The evaluated sensor yielded a good and reliable LAI estimates under maize canopy. [ABSTRACT FROM AUTHOR]

This study investigates the future climatic impacts of different percentages of trees/shrubs, C4 and C3 plant functional types (PFTs) over the West Africa region. The ratio of co-existence among the different PFTs was done as a representation of agri-silviculture practices over the region. Nine sensitivity experiments of different percentages of trees/shrubs, and C4 and C3 PFTs were carried out with a regional climate model (RegCM4) driven by Global Climate Model (HADGEM2-ES) outputs. These experiments were carried out along the Guinea Savana zone of West Africa using both prescribed and dynamic vegetation options of the model. The model simulated the seasonal evolution of precipitation and temperature fields quite well, with correlations greater than 0.8, but exhibited cold and wet biases of about 1–2 °C and 1–4 mm/day, respectively. Widespread warming (1–3 °C) and drying (1–2 mm/day) is projected in the near future across most parts of West Africa all year round. The West African future climate change associated with the different percentages of trees/shrubs, and C4 and C3 PFTs varied with the vegetation state (prescribed or dynamic) and model domain sizes. The prescribed vegetation experiments induced cooling of about 0.5–2 °C in most areas along the designated agri-silviculture zone, except Liberia and Sierra Leone. Similarly, enhanced precipitation occurred over most parts of Ghana and coastal parts of Nigeria (0.5–3 mm/day). On the other hand, the dynamic vegetation option did not exhibit pronounced changes in temperature and precipitation, except with a larger domain size. This study suggests the implementation of agri-silviculture as a mitigation and adaptation land-use practice across West Africa if drought-tolerant crops and the deciduous trees are adopted. [ABSTRACT FROM AUTHOR]