Configuring the ACRU model

1. Configuring the ACRU model Andy Pike School of Bioresources Engineering and Environmental Hydrology. University of Natal, Pietermaritzburg.

2. STEP 1: Define the Problem • The configuration will be determined by the problem at hand • Try and foresee the questions that might be asked in the future to pre-empt a further configuration at a later stage

3. STEP 2: Fieldwork • Fieldwork is essential to account for changes in land cover and catchment development which are not reflected in the traditional information sources • Field visits can often give the modeller an idea of the hydrological responses of the various subcatchments

4. STEP 3: Delimit the Subcatchments (1 of 4) • Catchment boundaries should be natural watersheds and should account for the following features: • Special points of interest • Abstraction points, effluent/irrigation return flows, point sources of pollution, water treatment plants, IFR sites

5. STEP 3: Delimit the Subcatchments (2 of 4) • Soils • Exposed rock, highly eroded areas, water repellant soils (hydrophobic soils), geology • Land cover • Wetlands, commercial and indigenous forests, land cover in pristine condition • Agricultural areas • irrigated and dryland cultivation, intensive/commercial agriculture, subsistence agriculture

6. STEP 3: Delimit the Subcatchments (3 of 4) • Rainfall • Catchments can be divided when a large variation in Mean Annual Precipitation is evident • Topography • slope • altitude • Impoundments • Major dams should always be at the outlet of a subcatchment

7. STEP 3: Delimit the Subcatchments (4 of 4) • Gauging stations and weirs • These need to be at the outlet of subcatchments in order for the simulated streamflows to be compared to observed data

8. STEP 4: Digitise and Number (1 of 3) • The subcatchment boundaries need to be digitised accurately and the areas need to be determined in km2 • Each subcatchment should be numbered in sequential order from the sources to the mouth • These numbers should be entered as a new field in the attribute table of the Shapefile

9. STEP 4: Digitise and Number (2 of 3) (from page AT2-13 of the ACRU Theory Manual)

10. STEP 4: Digitise and Number (3 of 3) • A utility (CreateMenuFromGIS) is available from the School of Bioresources Engineering and Environmental Hydrology to assist the users in configuration of catchments from ArcView (see http://www.beeh.unp.ac.za/pike/fortran/fortran_main.htm)

11. STEP 5: Rainfall • Selection of appropriate “Driver” rainfall stations • Identify all rainfall stations in the immediate area • Select the most appropriate “driver” station for each subcatchment (based on years of record, MAP, altitude, distance away from the subcatchment) • Infil missing records and make sure that they form concurrent periods • Check for problems of “phasing” • Calculate adjustment factors from catchment and station median monthly rainfall in order that the point rainfall data are more representative of the catchment’s rainfall • A utility (CALC_PPTCOR) is available from the School of Bioresources Engineering and Environmental Hydrology to assist the users in this process (see http://www.beeh.unp.ac.za/pike/fortran/fortran_main.htm)

12. STEP 6: Other Climate Information • Mean monthly A-pan data • Median monthly maximum and minimum temperatures • Daily maximum and minimum temperature data

13. STEP 7: Soils Information • Sources: • ISCW Land Type Database • SIRI 84 Homogeneous Soil Zones • ARC Biotopes • A utility (AutoSoils) which automatically assigns soil water retention and drainage characteristics to each ISCW Land Type is available from the School of Bioresources Engineering and Environmental Hydrology

14. STEP 8: Landuse Information • Sources: • Acocks’ Veld Types (follow “Tips and Tricks” link from http://www.beeh.unp.ac.za/acru/) • CSIR (Environmentek) National Land Cover (NLC) Database (click icons below) NLC1994/1995 NLC2000

15. STEP 9: Streamflow/Runoff Information • The following variables and parameters control the generation and timing streamflow: • stormflow response fraction for the catchment/subcatchment (QFRESP) • coefficient of baseflow response (COFRU) • effective (critical) depth of the soil (m) from which stormflow generation takes place (SMDDEP) • option to include or exclude baseflow from the simulation of streamflow (IRUN) • fraction of the catchment occupied by adjunct impervious areas (ADJIMP) • fraction of the catchment occupied by impervious areas which are not adjacent to a watercourse (DISIMP) • surface storage capacity (i.e. depression storage, or initial abstraction) of impervious surface (STOIMP) • option to simulate the water budget of an internally drained area (LYSIM) • coefficient of initial abstraction (COIAM)

16. STEP 10: Irrigation Information • Requirements: • Areas irrigated • Months during which irrigation occurs • Application rates and modes of scheduling (amounts and cycles) • Crop irrigated and their growth characteristics

17. STEP 11: Abstractions • Volumes and timing • Source (run-of-river or impoundment) • Return flows

18. STEP 12: Impoundments • Surface area • Volume • “Internal” (farm dams) or “external” • Environmental flow releases, legal flows and seepage • Evaporation

19. STEP 13: Verifications • Comparison of simulated flows to observed data (daily, monthly or annual) • Use: • Regression and comparative statistics • Time series plots • 1:1 plots • Double mass plots

20. STEP 14: Scenarios • Evaluate the impacts of changes in: • land cover • land use and management • operating rules • optimisation of irrigation scheduling • optimisation of dam sizing

21. Consult the ACRU Homepage for further information http://www.beeh.unp.ac.za/acru