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University of the Western Cape (2016)

Improving estimation of precipitation and prediction of river flows in the Jonkershoek mountain catchment

Mbali, Siphumelelo

Titre : Improving estimation of precipitation and prediction of river flows in the Jonkershoek mountain catchment

Auteur : Mbali, Siphumelelo

Université de soutenance : University of the Western Cape

Grade : Magister Scientiae - MSc (Earth Science) 2016

Résumé
Rainfall is the main input into the land phase of the hydrological cycle which greatly determines the available water resources. Accurate precipitation information is critical for mountain catchments as they are the main suppliers of usable water to the human population. Rainfall received in mountain catchments usually varies with altitude due to the orographic influence on the formation of rainfall. The Langrivier mountain catchment, a sub-catchment of the Jonkershoek research catchment, was found to have a network of rain gauges that does not accurately represent the catchment rainfall. As a result, this study aimed to improve the estimation of catchment precipitation and evaluate how improving estimation catchment precipitation affects the prediction of streamflows. Improving estimation of catchment rainfall in mountain catchments requires that a network of rain gauges that enables accurate estimation of the catchment rainfall is established. Establishment of such a network is problematic in mountainous areas such as Langrivier since some of the locations are not accessible and/or difficult to routinely visit for collecting rainfall data. Furthermore, rainfall is not the only form of precipitation in mountain catchments. Clouds directly contribute to precipitation in mountain catchments through cloud water interception. For most mountain catchments, the contribution of cloud water interception to total precipitation and consequently streamflows is unknown. Therefore, this study expanded the existing rain gauge network in Langrivier to include higher elevation areas of the catchment in order to improve estimation of catchment rainfall. The contribution of cloud water interception to total precipitation was assessed by monitoring cloud water precipitation along an altitudinal transect using Lovred fog screen gauges. Rainfall results revealed that the rain gauge network that was in place before it was expanded to higher elevation levels was underestimating catchment rainfall. Rainfall measured at 360-800 m.a.s.l altitudinal range in Langrivier is statistically similar at p = 0.05, with only the rainfall measured at 1214 m.a.s.l being significantly different from the rainfall at the 360-800 m.a.s.l altitudinal range. Cloud water contributes up to 35% to total precipitation at higher elevation areas. Cloud water is critical in the summer season ; cloud water contributed over 200 mm to the total precipitation at higher elevations in summer month December 2014. Having improved estimation of catchment precipitation, the ACRU model was used predict streamflow of the Langrivier. Improving estimation of catchment precipitation led to improved predictions of streamflow for the Langrivier catchment. While streamflow modelling was undertaken with precipitation input either being rainfall only or both rainfall and cloud water interception, the best simulation results were achieved by using rainfall only as precipitation input. Inclusion of cloud water interception to precipitation input led to ACRU over estimating streamflows. Cloud water does not directly contribute to streamflow hydrograph characteristics ; cloud water only leads to a reduction of evaporation rates. Configuring the model to be lumped or semi-distributed mode had an effect on modeling results. Ultimately, this study contributed improved understanding of the design of rain gauge networks, importance of cloud water to the Langrivier catchment and hydrological responses of small mountain catchments.

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