Table 2.

Technical details of West Midlands–Worfe groundwater model

Groundwater model typeMODFLOW 96 (McDonald & Harbaugh 1988)
Groundwater model layer typeTransmissivity calculated from saturated layer thickness, variable transition between unconfined and confined conditions (MODFLOW layer type 3)
Model layers, stratigraphically from top (Triassic) to bottom (Permian) with calibrated hydraulic parameters(1)Mercia Mudstone (confining layer, inactive)Kh* (m/day)Kv* (m/day)Sy*Sc*
(2)Upper Bromsgrove0.20.0030.12Embedded Image
(3)Lower Bromsgrove0.250.0030.12
(4)Upper Wildmoor1.50.0030.1
(5)Lower Wildmoor1.50.0030.1
(6)Upper Kidderminster2.50.0150.12
(7)Lower Kidderminster2.50.050.12
(8)Upper Bridgnorth2.50.250.14
(9)Lower Bridgnorth2.50.250.14
Grid spacing500 m
Stream boundariesMODFLOW stream cells (Prudic 1989)
Stream leakage coefficients
(m3/day/km)
Smallest streams/springs300
Moderate sized streams (Stour, Smestow Brook)1000
Large streams (Severn)2000
Other boundariesMODFLOW general head cells (McDonald & Harbaugh 1988) representing inflows from adjacent Carboniferous strata along faulted margin. Contributes 6.2% of total water balance
Starting headsSet heads followed by time variant run-in, 1380–1899 decadal stress periods, 1900–1959 annual stress periods, January 1960–December 1969, monthly stress periods
Validation (historic) time seriesJanuary 1970–July 2006, monthly stress periods
Abstraction (end validation time series)220 Ml/day (220 000 m3/day)
Runoff–recharge model type4R Code (Heathcote et al. 2004)
Soil moisture calculationPenman–Grindley (Grindley 1967)
Soil moisture calculation time stepDaily, recharge aggregated to monthly stress period
Type of unsaturated zone recharge transfer function4 days delay per metre of unsaturated zone thickness
Start of records used for calibrationHeadGauged stream flow
Pre 196020
1960–197042
1970–1990373
Post 199030
Acceptability criteria – calibration (see Soley et al. 2001 for detailed discussion)For head – average difference ±5 m; 80% within ±10 m; 50% within ±5 m; within ±3 m close to streams; trends matchedFor flow ±15%; trends matched
Mass balance errorMaximum of 0.02% of total flow balance
Predictive simulation starting headsJuly 2006
Equivalent predictive time seriesAugust 1970–July 2006
  • *Kh, Horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; Sy, specific yield; Sc, confined storage.