Modelling Runoff Estimation In Bida Basin Using Improved Soil Moisture Balance Model

Abstract

In this paper, an improved soil moisture balance model was used in estimating surface runoff in Bida basin. The model is a single layer soil moisture balance that estimates daily surface runoff using daily rainfall, some hydrological parameters coupled with both soil and crop parameters. The model estimates were validated with discharge measurements using field measurement of direct runoff using weir. This was used in calculating the runoff coefficients of the basin with the aid of soil moisture deficit and rainfall depths. Five (5) years, ranging from 2015 to 2019 were used as study years of which daily rainfall depths and daily minimum and maximum temperature data were used in the model validation. From the results obtained, the annual runoff from the basin for 2015 water year is 507.85 mm, which is 39.3 % of the annual rainfall. For 2016 water year, the annual runoff from the basin is 501.64 mm, which represents 34.05 % of the annual rainfall of 1473.2 mm. The first recorded significant runoff was observed between 125th and 175th days in all study years (when SMD = 0) while the highest value was recorded at 180th day.as 47 mm. 2018 study years recorded the highest annual rainfall depth as 1615.2 mm, while the year 2019 study year recorded the highest annual runoff value as 580.76 mm, which is observed as 41.30 % of annual rainfall. The highest values of runoff depth recorded in the years 2018 and 2019 study years could be attributed to the lower value of SMD during those years as against the higher values recorded in the years 2015 to 2017. Once the hydrological parameters (crop and soil properties) involved like TAW, RAW, SMD, PE, ETo and especially runoff coefficients can be selected carefully; a reliable estimation of hydrological outputs like runoff, recharge and rainfall-runoff relationship can be achieved with high precision.