Applying MODFLOW to wet grassland in-field habitats: a casestudy from the Pevensey Levels, UK

Abstract

Historical drainage improvements have created complex hydrological regimes in many low-lying, wet coastal grassland areas. The manipulation of ditch water levels is a common management technique to maintain important in-stream and in-field habitats in such areas. However, in wet grasslands with low soil conductivities the water table in the centre of each field is not closely coupled to variations in ditch stage. Consequently rainfall and evaporation have a greater influence on the depth to water table and water table fluctuations within each field. In-field microtopographic variations also lead to subtle variations in the hydrological regime and depth to water table that create a mosaic of different wetness conditions and habitats. The depth, duration, timing and frequency of flooding from accumulated rainfall, surface water and standing groundwater also influence the availability of suitable in-field habitats. Land drainage models are often used for studies of wet grasslands, but tend to be more complex and require more field variables than saturated zone models. This paper applies a 3D groundwater flow model, MODFLOW, to simulate groundwater levels within a single field in a wet coastal grassland underlain by a low permeability sequence and located in the central part of Pevensey Levels, Sussex, UK. At this scale, the influence of vertical leakage and regional groundwater flow within the deeper, more permeable part of the sequence is likely to be small. Whilst available data were not sufficient to attempt a full calibration, it was found that the sequence could be represented as a single, unconfined sequence having uniform hydraulic properties. The model also confirmed that evaporation and rainfall are the dominant components of the water balance. Provided certain information requirements are met, a distributed groundwater model, such as MODFLOW, can benefit situations where greater hydrological detail in space and time is required to represent complex and subtle changes influencing the in-field habitats in wet grasslands with low permeability soils.

Irrigation efficiency and shallow groundwater in anisotropic floodplain soils near Lake Tana, Ethiopia

“…The S y values investigated by all three methods (Table ) are within the range of typical literature values. For instance, Bradford and Acreman () listed the S y values for alluvial deposits to range from 1 to 10% (mean 6%) for clays and 10–30% for silty sands. Due to variability of the S y values even within the same textural classes, Healy and Cook () recommend using smaller values for better recharge estimation.…”

“…For instance, the lower horizon in field 1 (B 2 113-160 cm), which has 59% sand and 16.2% clay, has the highest S y (11.5%) in this method. For instance, Bradford and Acreman (2003) listed the S y values for alluvial deposits to range from 1 to 10% (mean 6%) for clays and 10-30% for silty sands. The S y values obtained by moisture-tension and mechanical-analysis methods were also generally related to sand content (Tables II and III).…”

Effective groundwater-surface water exchange at watershed scales

“…where ET plays a significant role (e.g. Bradley, ; Bradford and Acreman, , Wang et al ., ; del Pilar Alvarez et al ., ). This relationship is more appropriate for the common situation where there are insufficient data to support a more complex treatment of the ET–WT depth relationship.…”

“…However, at the scales considered here, where it is recognized that this relationship is likely heterogeneous and difficult to characterize, we revert to the linear relation used successfully in a number of regional modelling studies, e.g. where ET plays a significant role (e. g. Bradley, 2002;Bradford andAcreman, 2003, Wang et al, 2008;del Pilar Alvarez et al, 2012). This relationship is more appropriate for the common situation where there are insufficient data to support a more complex treatment of the ET-WT depth relationship.…”

Spatially distributed modelling of surface water-groundwater exchanges during overbank flood events – a case study at the Garonne River

“…As they allow characteristics of the environment to be taken into consideration on a detailed scale, physically-based, spatiallydistributed models are valuable tools for improving knowledge of SW-GW exchanges and assessing the three-dimensional nature of the problem ( Sophocleous, 1998, Bradford andAcreman, 2003 ). Numerous studies featuring distributed models and including SW-GW exchanges in floodplain areas have been carried out, encompassing SW-GW exchange patterns and biogeochemical zonation induced by meander sinuosity ( Boano et al, 2006, Boano et al, 2010, heat transport ( Brookfield et al, 2009, Horritt et al, 2006, 3D-flow patterns in relation to river level variations ( Derx et al, 2010, Nützmann et al, 2013, hydrofacies heterogeneity impact on SW-GW exchanges ( Frei et al, 2009 ), SW-GW exchange impact on floodplain water balance , solute transport through riparian areas , Hoffmann et al, 2006, Peyrard et al, 2008 and contribution of GW to stream water ( Partington et al, 2011 ).…”