GROUNDWATER

GROUNDWATER

1. The valuation of groundwater recharge is a select parameter in hydrogeology, difficult to find, which appreciates and tests various approaches. The methods are direct, geochemical, physical and climatic.

   • The climate method or water balance method is used due to its ease of use and the availability of information (Tabue Youmbi, 2013; Fontang et al., 2016; Kouassy Kalédjé et al., 2016).
   • It is based on the principle that precipitation (P) that falls in a given region is divided between evapotranspiration (ETR), groundwater recharge (I) and surface flow (R), the hydrological balance equation is stated by the relationship P=ETR+R+I (1)
     • Since rainfall is a known data, the ETR is made explicit by the Coutagne method (the Turc and Thornthwaite methods do not allow establishing the hydrological balance in this type of region as Djeuda Tchapnga et al. (1999) and R (runoff) by the Tixéront-Berkaloff method.
   • 2.2.1 How to establish groundwater recharge

2. 2.2.2 Estimation of potential evapotranspiration and actual evapotranspiration

   • Thornthwaite formulated an expression to assess FTE based only on monthly temperature (Djeuda Tchapnga et al., 2000; Kouamé et al., 2011; Lachassange et al., 2015; Kouassy et al., 2016; Fantong, 2016).
     • ETP-16(10T/1)^a x F(λ) (2) Avec: α=0.49239+〖1,79.10〗^(-2) I-7,71.10^(-5) I^2+6,75.10^(-7) I^3;I= ∑_(j=1)^12▒〖i ou i=(T/5) 〗^1,514
       • T: mean monthly temperature in °C; I: annual thermal index; i: monthly thermal index; F: correction coefficient, function of latitude and given month. Regarding ETR, the following Coutagne formulation has been maintained (Lasm, 2000; Kouamé et al., 2011; Kouassy et al., 2016): ETP=P-〖λP〗^2 (3) Here, λ=1/(0.8+0.145 T ; T: mean annual temperature in °C; P: average annual rainfall in m. The study of this formula is limited by the following relationship:
         • 〖V8〗^μ<P<V2^μ Cuando P<1/8^μ ETR=P Si P>〖1/2〗^μ ETR=0,20+0,035T

3. 2.2.3 Runoff estimation

   • After illustrating a set of basins, Tixéront and Berkaloff established a correlation whose simplified form gives runoff (Kouassi et al., 2012; Onana et al., 2017; Jalloh et al., 2018).
     • R=P^2/〖3ETP〗^2 (4) With R: runoff in mm; P: average annual precipitation in mm; ETP: average annual potential evapotranspiration calculated by the Thornthwaite method in mm.

4. 2.2.4 Infiltration estimation

   • It is found by difference with the other parameters, that is to express
     • I=P- (ETR+R)