Rainfall is a key contributing factor to land degradation such as soil erosion. This is as a result of the ability of rainfall to dissolve, loosen or worn away soil by the force of raindrops, runoffs, and river flooding and deposit in other places (Balogun et al. 2012; World Meteorological Organization 2005). Generalized maps of the geographical distribution of rainfall and wind erosion, positions Ghana in the area predominantly vulnerable to rainfall erosion. The demand for land and agricultural products due to population growth is likely to aggravate the problem (Oduro-Afriyie 1995; Norman 1981).
The rate at which the soil at the Ghana Atomic Energy Commission (GAEC) site is eroded is of much concern as some of the nuclear installations of the Commission are underground and requires the soil to be conserved. Ghana is implementing the Borehole Disposal Concept (BDC), a specially engineered borehole, 30–100 m deep with narrow diameter (0.26 m) designed to dispose disused radioactive sources of less than 110 mm in length and 15 mm in diameter. This concept was developed in South Africa under the International Atomic Energy Agency’s (IAEA) AFRA project for the disposal of disused sealed radioactive sources (DSRS) in member states with relatively small DSRS inventories.
To perform the Post Closure Safety Assessment on the proposed Borehole Disposal Facility (BDF) for Ghana, one of the key parameters that needs to be investigated at the proposed site is the surface erosion rate. This parameter will enable us ascertain the duration for the closure zone (the zone between the disposal zone and the ground surface) of the BDF to be eroded for the disposed waste to be uncovered. To be able to compute the surface erosion rate, the rainfall erosivity index for the site is prerequisite hence the need to compute this parameter.
Rainfall erosivity is a function of its amount, duration, drop size and drop size distribution, terminal velocity, intensity and kinetic energy. The significance of rainfall erosivity in the assessment of soil erosion risks stems from the fact that, unlike other natural factors that affect soil erosion, the erosive capacity of rainfall is not subject to human modification (Balogun et al. 2012; Anugulo-Martinez and Begueria 2009; Salako 2003).
Rainfall has an erosive force that is expressed as rainfall erosivity. Rainfall erosivity ruminates the rainfall amount and intensity mostly stated as the R-factor in the universal soil loss equation (USLE) and its revised version, RUSLE (Panagos et al. 2015). Due to scarcity of data to estimate the R-factor, this study estimates the rainfall erosivity in the investigated area using rainfall data.
Oduro-Afriyie (1995) used the Fournier (1960) index (FI), defined as:
$$\frac{{P_{max}^{2} }}{P}.$$
(1)
\(p_{max}\), the rainfall amount in the wettest month and P is the annual rainfall amount, to compute rainfall erosivity indices for various stations in Ghana. This FI index has limitations as an estimator of the rain erosivity index because low amounts of monthly rainfall can have erosive power. It is irrational that if the maximum monthly rainfall \(p_{max}\) remains the same with an increase mean annual rainfall, the (FI) decreases since an increase in total rainfall should result in an increase of erosivity (Deyanira and Donald 2005).
Based on these limitations, Arnoldus (1980) modified the (FI) index into a modified Fournier index (MFI) including the amount of rainfall of all the months in the year:
$$MFI = \mathop \sum \limits_{i = 1}^{12} \frac{{p_{i}^{2} }}{P}.$$
(2)
with p
i
the monthly rainfall amount for the ith month (mm) and P: the annual rainfall amount (mm), Lujan and Gabriels (2005). This study, therefore, estimates the rainfall erosivity of the GAEC site using the Modified Fournier Index, Arnoldus (1980).
