Integrating rainfall runoff and evaporation models for estimating soil water storage during fallow under in-field rainwater harvesting
Zerizghy, Mussie Ghebrebrhan
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English: In-field rainwater harvesting (IRWH) is a beneficial water conservation practice. Fallowing is an important strategy to enhance water conservation. Fallowing consists of a period where no crops are grown (bare) and the field is kept free from weeds to stop unproductive transpiration losses. In such a system, to determine the rainwater storage under the profile of the basin the processes of in-field runoff and evaporation are very important. Until the storage capacity for the soil is exceeded these two processes are the sole determinants of the rainwater received. In this study, it was hypothesized that it will be possible to characterize the water balance for an IRWH system by integrating rainfall- runoff and evaporation models and estimate water storage during fallow period. The field, prepared for IRWH, was kept fallow and soil water evaporation and in-field runoff observation experiments were conducted. The IRWH plots were prepared in three basin to runoff strip length (RSL) ratios: 1:1, 1:2 and 1:3. The study was conducted on the Kenilworth/Bainsvlei and Paradys/Tukulu ecotopes. These ecotopes share the same climate and topography, but have different soil types. Rainfall characterization was done by using long-term rainfall data. Rainfall event amount, duration and intensity classification with corresponding percentages of representation were obtained. This classification is important in producing relevant rainfall simulations. Rainfall simulations to observe the effect of rainfall intensity pattern on runoff amount and infiltration progress were conducted. The results showed that the rainfall intensity pattern had a significant effect on the runoff amount for the Paradys/Tukulu ecotope, but not for the Kenilworth/Bainsvlei ecotope. The advance of the infiltration front, despite the clay content difference of the top horizon of the two ecotopes, revealed that it only affected the top 200 mm. Having observed that the amount of simulated rain tops the majority of rain events received in these ecotopes, this shows that rainwater infiltration and runoff are mainly affected by the top horizon of the soil. Thus, the infiltration progress during rainfall event, for an IRWH system, can be grossly categorized according the top horizon considered. Soil water evaporation is an important parameter that decides the fate of the received rainwater. Hence, accurate quantification of this parameter is of paramount significance. The neutron water meter and DFM capacitance probe were compared to measure soil water evaporation from the top 300 mm. It was found that the capacitance probes performed better. Thus capacitance probes were used to measure soil water evaporation across different sections of the micro-landscape created by the IRWH system. These measurements were used to compare evaporation across different sections of the micro- landscape and among different basin to RSL ratios. There were significant differences in the evaporation observed across the sections of the micro-landscape. The plot level evaporation comparison between the different basin to RSL ratios, however, did not show significant differences. Modelling of in-field runoff and soil water evaporation was done by selecting two models each research field. For the runoff modelling, an empirical runoff model was developed and compared to the Morin & Cluff (1980) (MC) runoff model. The models were calibrated for each basin to RSL ratio. Both models showed good prediction performance on validation data. The RMSE values for the empirical runoff model were 5.9, 4.2 and 7.1; 4.0, 3.8 and 5.5 mm for the 1:1, 1:2 and 1:3 basin to RSL ratios of Kenilworth/Bainsvlei and Paradys/Tukulu ecotopes, respectively. These values for the MC model were 6.9, 5.1 and 8.5; 6.4, 7.8 and 9.2 mm, respectively. It was concluded that the empirical runoff model performed better than MC model on both ecotopes. Similarly, an empirically developed evaporation model and Ritchie (1972) evaporation prediction (REP) model were calibrated for both ecotopes and different basin to RSL ratios. The empirical model related cumulative evaporation to the square root of days after rain and square root of cumulative reference evaporation. The validation of the empirical evaporation model showed that the RMSE values for the 1:1, 1:2 and 1:3 basin to RSL ratios were 0.74, 2.7 and 3.5; 1.0, 1.8 and 4.3 mm on the Kenilworth/Bainsvlei and Paradys/Tukulu ecotopes, respectively. These values for the REP model were 1.1, 2.5 and 3.2; 1.2, 1.6 and 3.2 mm, respectively. The model performance varied on the two ecotopes. Overall, the empirical evaporation model performed better on the Kenilworth/Bainsvlei ecotope, while the REP model performed better on the Paradys/Tukulu ecotope. The best performing runoff and evaporation models were integrated in a soil water balance exercise during a fallow period. The water balance was conducted for long (18 months) and short (6 months) fallow periods. The plot level RSE values ranged from 8 to 33% and 29 to 58% for the long and short fallows, respectively. These ranges for the Paradys/Tukulu were 7 to 24% and 23 to 56% for the long and short fallows, respectively. For long fallow the storage gains achieved were not different among the different basin to RSL ratios. For the short fallow, however, the storage gains increased with increasing RSL.