Wetland Water Balance Linking hydrogeological processes to ecological effect

Transcription

1 Wetland Water Balance Linking hydrogeological processes to ecological effect Don Ross & David Gasca Outline Background Wetland-scale processes Water balance approach Hydrogeological inputs & ecological assessment Summary

2 Background Approach based on existing methods and research Armstrong (1993) Gowing and Spoor (1998) Gasca & Acreman (1999) EA/NE Hydro-ecological guidelines (2004), etc Aim: to integrate these into a practical and adaptable tool Habitats Directive assessments WLMPs Habitat Creation. Illustrated by: Arun Valley SPA Sustainability Study Wetland-scale processes Evapotranspiration Precipitation Seepage through embankment Sluice discharge Marginal seepage Stream inflows River inflows Outflows to & pumping from south catchment Upflow from or leakage to aquifer

3 Water balance: open water 15,000 10,000 Volume (m 3 ) 5,000 0 Inflows from River Arun Rainfall-runoff -5,000 Stream inflows Marginal seepage Pumping Ground-Surface Water Interactions Sluice discharge Evaporation Sluice leakage -10, Open water: volumes to levels Cross-sectional area x ditch length Cross-sectional area x ditch length (Cross-sectional area x ditch length) + Field storage (from DEM) WL1 WL2 WL1 WL3 WL2 WL1 WL3 Storage (S) WL1 WL2 S = aw 2 + bw + c Water level (W)

4 PB001 PB003 PB004 PB006 PB007 PB010 PB013 PB023 Mean CSA PB002 PB005 PB011 PB015 PB016 PB025 Mean CSA PB008 PB009 PB012 PB014 PB019 PB024 PB026 Mean CSA PB017 PB018 PB020 PB021 PB022 PB027 PB028 Mean CSA Open water: volume-level data needs For over-bank storage: DEM For ditches and permanent ponds/lakes: topographic transects 2.0 a) Type 1a 2.0 c) Type Level (m AOD) Level (m AOD) Width (m) Width (m) 2.0 b) Type 1b 2.0 d) Type Level (m AOD) Width (m) Level (m AOD) Width (m) Ditch field water table interactions E P R -Qd +Qd V (Armstrong, 1993; Swetnam et al., 1998) M L t-1 L D L Includes water-table level dependent evapotranspiration (same basis as MODFLOW EVT)

5 Open water model calibration Open water model calibration plot (actual vs modelled) Open water model parameters Sluice level data used in historic predictions Open water model sensitivity buttons Historical model prediction Water table model parameters Buttons linking to other parts of the model Water table model sensitivity buttons Quality control of open water model output Water table model calibration With groundwater models it is hardly ever possible to get to this type of wetland water table representation

6 Hydrogeolgical inputs and ecological assessment e.g. Arun Valley SPA, Pulborough Brooks SSSI Pulborough Brooks Hydrogeology B N A Groundwater Level (m AOD) Empirical Groundwater Level Predictions Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06 Jan-07 SSO_08 SSO_03 SSO_10 PB w est prediction PB east prediction Recharge signal SSO_05

7 Ditch level sensitivity to seepage rates High seepage case Impact of High and Low seepage Predicted casehistoric Seepage Rates on Calibration of the Ditch Model Actual ditch water level 1.90 Water Level (m OD) See detail in chart below Oct-05 Dec-05 Feb-06 Apr-06 Jun-06 Aug-06 Oct-06 Dec High seepage case Low seepage case Actual ditch water level 1.60 Marginal seepage GW levels predicted by empirical model, but, rates of seepage uncertain. Attempts to measure rates unsuccessful/equivocal. Sensitivity studies using wetland model used to constrain upper limit of seepage rate. Water Level (m OD) Jun Jul Jul Aug Aug-06 Source file: _Arun_SPA\60_Work processes\task8 Quantitative assessment tools\wetland Assessment\Model results Simplified water balance results 80,000 40,000 (a) Ditch water balance Rainfall-runoff Stream inflows Inflows from River Marginal seepage Sluice discharge Evaporation Sluice leakage Water table interaction Pumping 0-40,000-80,000 Oct-05 Nov-05 Dec-05 Jan-06 Feb-06 Mar-06 Apr-06 May-06 Jun-06 Jul-06 Aug-06 Sep-06 Oct-06 Nov-06 Dec-06 Jan-07 Volume (m 3 )

8 Hydro-ecology assessment Wet grassland Water table requirements expressed in terms of long term average water table depths (bgl) for months through the year Based on monitoring data from research sites, regimes for different communities (MG13 in our case) identified: Green: target regime Amber: if experienced for most years community change is likely Red: if experienced one year community change is likely Hydro-ecology assessment Ditch communities (a) Cross section 7 Minimum tolerable water depth Water depth Water depth (m) (b) Cross section 13 Minimum tolerable water depth Water depth Water depth (m) LOW SEEPAGE SENSITIVITY CASE Abstraction Scenario Deep Ditch cross section Shallow S1 S5 S6 S7 S8 S12 S13 S2 S3 S4 S9 S10 S Actual Zero Fully licensed (FL) FL, sluice leakage at 50%

9 Applications Chippenham Fen Groundwater abstraction Heath Lake Hydrology and water quality study Hilsea Lines Fisheries and water quality study Hale Manor Farm Reservoir design Wallers Haven South East Water Surface water abstraction Pevensey Levels Water Level Management Plan Pulborough Brooks Southern Water Groundwater abstraction Bembridge Lagoons Hydrology and water quality study Summary Focus on wetland scale processes Understanding gained from12-18 months monitoring data can be hindcast over longer periods Ecological requirements can be specified and compliance assessed Use wetland water balances to bridge the gap from hydrogeological impacts to ecological effects! If you are interested in the approach, contact: or