RAINFALL RESPONSE TO DAM/IRRIGATION PROJECTS IN NORTHERN NIGERIA

In this paper, we examine the possibility that the increasing number and size of dam/irrigation projects in northern Nigeria are having a corresponding increase in rainfall in spite of the threat of climate change. We modeled the rainfall trends over 11 meteorological stations over a period of 34 years (1971 - 2004). The trends were compared with each other based on the closeness of the meteorological stations with dam/irrigation projects. Results obtained showed a compelling evidence of a direct relationship between rainfall and dam/irrigation projects. The results indicated that the average rate of change in rainfall trends for the 11 stations ranged from – 4.08 to 24.83mm per annum. Kano area had 24.83mm while Jos area had - 4.08. Results obtained in this study thus provide evidence as to the extent that rainfall has responded to dam/irrigation projects.


INTRODUCTION
Temperature which is presently increasing globally is not only an element of climate and weather but also a factor of climate.As a factor, any change in it is nearly always reflected in changes in the other climatic elements.For instance, an increase in temperature can result in dryness in some areas, and for others it could result in increased rainfall.As temperature increases, evapo-transpiration also increases.In other words, the molecules of water that leave from plants and other wet surfaces such as dams, lakes, rivers, seas, oceans, etc., to the atmosphere increase with increasing temperature.With this, more molecules of water are added to the atmosphere and such will encourage condensation and subsequently and/or possibly precipitation.
The present increasing temperature of the globe commonly known as global warming has caused a change in the world's climate (Obasi, 1992;Barry and Chorley, 1992;Kaser, 1999;IPCC, 1996IPCC, , 2001;;Desanker and Justice, 2001;and Goudie, 2002).Basically, components of the climatic system, as with other systems, respond to perturbations either from within or from without the system.(Meyer 1996;Danielson et al., 1998;and Pidwirny, 2004).This implies that the other elements of climate should be changing in response to increasing temperature.Precipitation (rainfall in this case) is one of such elements being affected.For example , Nicholson, (2001) has noted a long-term reduction in the rainfall of semiarid regions of West Africa, and increased aridity in most of the Africa region particularly since the 1980's.Similarly, Obioha (2005) reported that the sahelian zone of north-eastern Nigeria has been experiencing a change in its climate characterized by reduction in rainfall, increase in the rate of dryness and heat.Meanwhile, rainfall has increased over the last 50years over the northwestern Australia (Australian Greenhouse Office, 2003).Looking at these two conditions, it could be speculated that the likely reason for the increased dryness in the Sahel regions of West Africa with increasing temperature is the influence of the G. N. Northern Nigeria is situated between latitudes 7 and 14 0 N and longitudes 3 0 and 15 0 E (Fig. 1).It has common borders with arid areas namely Niger Republic in the north and north-west, northern part of Benin Republic in the west, northern Cameroon in the east, with the exception of the south where it shares boundaries with southern Nigeria which is wetter.Northern Nigeria covers about 75 percent of the area of Nigeria.It has an area of about 692, 826 square kilometres and more than 400,000 square kilometres of it are arable land (Nnaji, 2001).
Sahara Desert which borders the area in the north.The Sahara desert is generally a dry area with little or no evaporation occurring over it to encourage rainfall.Conversely, the increase in rainfall over north-western Australia probably due to global warming could be attributed to the Indian Ocean and the moisture bearing wind from it that blows into the country through that part.
Dam/irrigation projects provide wet surfaces from which evapo-transpiration occurs.The number and sizes of such projects are increasing in Northern Nigeria (the study area) with increasing demand for food and water as the population increases.There is therefore the likelihood that these projects will have some impact on the project areas' rainfall.This paper therefore examines the extent that rainfall has responded to the dam/irrigation projects in Northern Nigeria.
The productivity of this arable land has been enhanced through the development of numerous dam/irrigation projects thus making it an important producer of much of Nigeria's food needs.The closeness of the northern fringes of the study area to the Sahara Desert and its south being bordered by the wetter 2. STUDY AREA southern Nigeria makes rainfall to vary significantly over the area from the southern part to its north.This is typified by Lokoja in the southern border with 7 months of rainy season and 200 days of rain, and Katsina in the extreme north with a rainy season of between 3 and 4 months, and 80 days of rain (Nnaji, 2001and Nwagbara, 2008).Equally, the average annual rainfall has a mean of more than 1200mm in the south and less than 300mm in the far north with exception in high altitude areas (Nnaji, 2001and Nwagbara, 2008).This rainfall pattern is the product of the interaction between the Tropical Maritime Airmass (mT) and Tropical Continental Airmass (cT) whose extent of influence depends on the position of the Inter-Tropical Discontinuity (ITD).The forth and back movements of the ITD over Northern Nigeria determine the Airmass whose characteristics prevail over the region.As a result, when the mT dominates, it brings rain due to its source region, the Atlantic Ocean, while dry season with harmattan comes from the dominance of the cT which originates from the Sahara Desert.The dry effect of the cT is often much to the extent that only dam/irrigation projects keep crops alive.

Data Used
Monthly rainfall data covering 34 years (i.e.1971 to 2004) for 11 meteorological stations in the study area (Fig. 2) were collected for this study.There were no missing data as data for the 4488 months in the 34 years for the 11 stations were available.The data were sourced from the Nigeria Meteorological Agency, Oshodi, Lagos.The 11 meteorological stations namely, Gusau, Jos, Kaduna, Kano, Maiduguri, Makurdi, Minna, Nguru, Sokoto, Yola and Zaria were selected based on their spatial distribution in the three major climatic regions of the study area: Tropical Hinterland, Tropical Continental North and High Plateau, and completeness of data required.

Technique of Data Analysis
The linear regression statistical technique was used to model the trends in rainfall data over the period of 34 years in the stations and study area as a whole.In applying this statistic to the data, the equation of the least squares line was first of all determined and then plotted.This technique can be expressed in the form: Results obtained from the application of the statistical technique were used to compare with each other based on stations and the extent of dam/irrigation projects around them.

RESULTS
Table 1 shows the results of the application of linear regression to annual rainfall totals.The results are further shown as linear trends in Figs.3A -D  The relationship between rainfall trends and some dam/irrigation projects in Northern Nigeria is presented in Table 2.  2, reveals that Kano has the highest trend slope (i.e.24.83mm), and incidentally has the highest number of earthfilled dams in operation with Tiga dam as the largest (NEST, 1991) among the stations studied.The surface area of the reservoir behind the Tiga Dam has a surface area, at full capacity, of 178 square kilometres (NEST, 1991).Incidentally too, Maiduguri area with comparatively fewer number of Dam/irrigation projects has a rainfall trend slope of only 1.79mm.

DISCUSSION
Massive dam and irrigation projects are becoming common in Northern Nigeria.In general, all the major rivers in Nigeria have been dammed or about to be dammed for one purpose or the other or a combination of them e.g.irrigation, flood control and power generation.Though dam and irrigation projects are usually a source of water molecules in the earth's atmosphere, the increase in global temperature, and sizes and number of these projects in Northern Nigeria seem to have accelerated the rate of evapo-transpiration.Ordinarily, evapotranspiration increases with increasing atmospheric temperature.The 24.83mm average rate of increase in the annual rainfall total of Kano is indicative of the influence of these projects on rainfall.This is because massive dams as found around Kano in the face of global warming are bringing about increased evaporation since larger evaporation surface is created by the dams.Similarly, the irrigation projects in the face of increased world temperatures are causing an increase in evapotranspiration in the sense that such projects increase the leave-area as plants increase in number, and the wet surface available.With this, the tendency for condensation to take place and cloud formed under this circumstance is very high.Areas of fewer and smaller dam/irrigation projects or none at all have a relatively far lower average rates of increase in their annual rainfall totals e.g.Maiduguri with 1.79mm and in the same climate region with Kano i.e.Tropical Continental North.

CONCLUSION
The first goal of the Millennium Development Goals (MDGs) is to eradicate extreme poverty and hunger by 2015.The increasing rainfall totals in Northern Nigeria which is attributable to increasing world temperatures and number and sizes of dams in the area (Chima and Nwagbara, 2007;and Nwagbara, 2008) is a positive climatic event which if tapped will bring about higher agricultural productivity.Through this, poverty will be substantially reduced in the area.This is because the population is still predominantly agrarian and dominated largely by small-holder subsistence farmers (Nnaji, 2001) who depend largely on rainfall for the survival of their crops and animals.With this positive role of massive dam/irrigation projects in encouraging rainfall, more of such projects should be developed by governments at the federal, state and local government levels.In doing this, mitigation measures against flooding of adjoining settlements and farmlands must be taken into consideration.The very large population of Nigeria and its "big-brother" role in Africa make it expedient to encourage irrigation farming especially as Africa as a whole has been identified as having very little irrigation, less than 5 percent compared to more than 30 percent of the land in Asia (Hassan, 2008).When more food is produced through the enhancement of more water from rainfall and dams/irrigation projects, food security will be guaranteed not only for the region but for Nigeria as a whole and even Africa, thus eradicating hunger in these areas.

Fig
Fig.1: Study Area (Northern Nigeria): Vegetation Zones and Selected Meteorological Stations

n
Σxy intercept; b the regression coefficient (slope); y the rainfall value; x the time in years; x the mean time; and y the mean rainfall values.

Chima ,
Department of Geography and planning, Abia State University, Uturu M

.O. Nwagbara, Department of Geography and planning, Abia State University, Uturu M. A. Ijioma, Department of Geography and planning, Abia State University, Uturu Editor's Note: This
paper was earlier Published in Vol. 8 (1) 2009.It is being republished here because of figures which were not cleared.

Table 1 : Intercepts, Slopes and Regression Line Equation for Annual Rainfall Totals
, 4A -D, and 5A -D.In Table1, only Jos station out of the 11 and General (i.e. the stations are grouped as one to represent the study area) stations possesses negative slope.It implies that all the other.