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

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

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

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

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

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

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

STEP 4: Digitise and Number (1 of 3) The subcatchment boundaries need to be digitised accurately and the areas need to be determined in km 2 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

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

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

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

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

STEP 7: Soils Information Sources: – ISCW Land Type Database ISCW – 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

STEP 8: Landuse Information Sources: – Acocks’ Veld Types (follow “Tips and Tricks” link from – CSIR (Environmentek) National Land Cover (NLC) Database (click icons below) NLC1994/1995 NLC2000

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)

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

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

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

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

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

Consult the ACRU Homepage for further information

Step 3 (1 of 4) – Include a digitised topomap