Browsing by Author "Rushton, Kenneth R."

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    Evaporation from bare soil: Lysimeter experiments in sand dams interpreted using conceptual and numerical models
    (Elsevier, 2018-07-06) Quinn, Ruth; Parker, Alison; Rushton, Kenneth R.
    Unlike evaporation from open water, the magnitude of evaporation from bare soil decreases as the water table falls. Bare soil evaporation studies have included field and laboratory experiments, mathematical formulations and semi-empirical models. However, there is only limited field information, especially concerning evaporation from bare sand. The semi-empirical approach of the FAO1 Irrigation and Drainage Paper 56, which contains guidelines for computing crop water requirements, can be adapted for bare soil evaporation with a three stage process. The suitability of the FAO 56 approach for bare sand evaporation is investigated by installing lysimeters in sand dams. Sand dams are shallow groundwater storage systems, which are designed on the assumption of reduced evaporation as the water table falls. The field results from the lysimeters are simulated adequately by a water balance model based on FAO 56 with an additional component to represent both the difference between the variable saturation with depth, which occurs in practice, and the assumption in standard water balance models of a sudden change from dry to fully-saturated conditions at the water table. This study demonstrates and quantifies the reduction in bare soil evaporation compared to open water or cropped areas and confirms the validity of the three stage FAO semi-empirical approach.
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    An examination of the hydrological system of a sand dam during the dry season leading to water balances
    (Elsevier, 2019-06-13) Quinn, Ruth; Rushton, Kenneth R.; Parker, Alison H.
    To address water scarcity in semi-arid regions, rainfall and runoff need to be captured and stored locally before they are lost to the sea. This can be done using a sand dam which consists of a reinforced wall constructed during the dry season across a seasonal riverbed. However it is unclear whether their main utility is to store water in the sand that is also trapped behind them, or to facilitate aquifer recharge. This paper aims to answer this question by the calculation of a water balance in three sand dams in Kenya to quantify the amount of water transferred between the sand dam and the surrounding aquifer system. The components of the water balance were derived from extensive field monitoring. Water level monitoring in piezometers installed along the length of the sand deposits enabled calculation of the hydraulic gradient and hence the lateral flow between the different reaches of the sand dam. In one sand dam water was gained consistently through the dry season, in one it was lost, and in the third it was lost almost all the time except for the early dry season in the upper part of the trapped sand. In conclusion sand dams should not be treated as isolated water storage structures.
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    Exploratory groundwater modelling in data-scarce environments : the shallow aquifer of River Yobe Basin, North East Nigeria
    (Cranfield University, 2002) Hassan, M.; Carter, Richard C.; Rushton, Kenneth R.
    This thesis addresses the issues of modelling a groundwater system in a data-scarce environment, the Yobe river basin, north east Nigeria. Despite significant investment in the past towards water resources developments, basic data on groundwater resources are limited. Short-term studies by Consultants contain some weaknesses and have not fully investigated the mechanisms of flow to and from the aquifer. Fieldwork studies conducted during this work and in the past (Alkali, 1995) showed that the shallow aquifer system is hydrogeologically complex. Concerns such as the relative magnitudes of recharge mechanisms to the aquifer, hydrologic conditions of the aquifer, a large change in river stage, presence of unconfined 'windows' for vertical recharge, and the fact that the region is located in a semi-arid region need to be addressed. This increased the concerns for the need to explore the system through modelling. Modeling can give insights into the whole system behaviour which other approaches cannot provide. Therefore modelling was carried out and it has provided valuable insights into the complex system. This thesis reports on the procedure of developing a groundwater model that is basic and exploratory based on limited data. Detailed conceptual model was developed using data from previous workers and from a fieldwork undertaken in this study. The conceptual model provided key hydrogeological information on the various physical processes and how they interact with the shallow Fadama alluvial aquifer. It describes the aquifer as around 10 m thick and about 4 km wide with the river partially penetrating it. The aquifer consists of areas that are confined and some that are unconfined. The river is ephemeral and its stage changes rapidly over 4 m. Recharge mechanisms to the aquifer consist of vertical recharge from rainfall and overland flooding through permeable topsoil, river to aquifer flow and 'leakage' through low permeability cover. The conceptual model was idealized and translated into a computational groundwater model using MODFLOW. The model investigated the role of each components of flow in determining the overall water balance of the system. The relationship between river stage and river coefficient in the study of river-aquifer interaction was investigated. Finally the response of the aquifer system to pumping was explored. Groundwater head output from the model was used in the calculation of the various flow components. The main findings and conclusions of the work are that: (i) a comprehensive conceptual model is fundamental in developing a numerical groundwater model; (ii) the exploratory model developed using limited data is plausible because it is hydrologically credible and fits the available data; (iii) the water balance shows that the river to aquifer flow dominates the recharge from rainfall and overland flooding. Contrary to initial belief, the largest river to aquifer flow occurs before the river reaches its peak; (iv) flows between river and aquifer are insensitive to variation of river coefficient with river stage. The limiting factor in the exchange of water between them is the hydraulic gradient and the transmissivity of the aquifer; (v) in representing the river with a constant river coefficient, the coefficient has a threshold value above which the river-aquifer interaction does not change significantly; (vi) over-pumping of the aquifer will decrease river flow to disadvantage of downstream users; (vii) the replenishment of the aquifer can be improved by pumping it at a modest rate.
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    Understanding the groundwater system of a heavily drained coastal catchment and the implications for salinity management
    (Cranfield University, 2007-05) Simpson, Trevor Baylie; Holman, Ian P.; Rushton, Kenneth R.
    The Thurne catchment in north-east Norfolk, UK, is an extremely important part of the Broads National Park, an internationally important wetland environment. Extensive engineered land drainage of the marshes of this low-lying coastal catchment over the past two centuries has led to land subsidence and the need for drainage pumps to control water levels sufficiently below sea-level to maintain agricultural productivity. Consequently, seawater from the North Sea has intruded into the underlying Pleistocene Crag (sand) aquifer and brackish groundwater enters into land drainage channels, thereby raising their salinity. Powerful pumps discharge these brackish drainage waters into a Special Area of Conservation (SAC) and RAMSAR site, leading to adverse ecological impacts on salt-sensitive species. Chloride concentrations within drainage channels throughout the network have been found to significantly vary, with several influential factors affecting channel salinity such as proximity to the sea and connectivity to the underlying aquifer. A thorough understanding of the surface-water/groundwater system and a subsequent quantification of the various processes has been necessary for the development for the drain/aquifer interactions and a numerical groundwater model. These models are used to estimate the long-term distribution of the salinity within the drainage system under current conditions. The model credibility is justified by comparable aquifer-drain water balance, a comparable coast water inflow/ total groundwater ratio and the particle tracking from the coastal reaches trace to previously-measured saline-vulnerable locations. The numerical groundwater model has demonstrated that the average daily inflow of saline groundwater into the Crag aquifer of the Thurne catchment is 3,081 m3/day, of which the HempsteadMarshes main drain is one of the main conduits for saline inflow into the Brograve system, which discharges directly into the SAC. Various changes to the engineering design or operation of the drainage system have been proposed to minimise the saline inflow to the SAC, but the implementation of any proposals must be considered in conjunction with the current dynamics of the system. Three separate management or engineering remedial measures have been modelled: (i) raising the water levels in the drains of the Hempstead Marshes in the north east of the catchment (ii) lining the main drain of the HempsteadMarshes with low permeability material, and (iii) The construction of a new coastal open ditch drain which is intended to ‘intercept’ the saline intrusion and prevent ingress into inland drains of the Brograve system. The results suggest that raising the water levels in the Hempstead Marshes will reduce the saline inflow into the Brograve sub-catchment substantially, and decrease the overall saline inflow into the Thurne catchment from 3081 m3/day to 2822 m3/day). The lining of the main drain in Hempstead produces a less than 10% decrease in saline inflow into the catchment from 3,081 m3/day to 2,958 m3/day. The simulated coastal interceptor drain could in theory through maintaining a low groundwater head near the coast, prevent the inflow of saline groundwater into the Brograve system. However, such a drain would increase the saline inflow across the coastal boundary by around six times (from 3,081 m3/day to 19,750 m3/day), remove large quantities of fresh groundwater from the Pleistocene Crag aquifer and lead to high energy and pumping costs. The research has shown that there are partial solutions to reducing the saline inflow into the drainage systems in this lowland coastal catchment. However, any intended alterations must first consider other potential impacts, such as changes to flood risk, land management restrictions or hydrodynamic effects on the receiving watercourse through changed discharge volumes.