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 theSubcatchments (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 theSubcatchments (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 theSubcatchments (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 theSubcatchments (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 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

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 http://www.beeh.unp.ac.za/pike/fortran/fortran_main.htm)

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)

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– 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

http://www.beeh.unp.ac.za/acru/)– CSIR (Environmentek) National Land Cover (NLC)

Database (click icons below)

NLC1994/1995 NLC2000

STEP 9: Streamflow/RunoffInformation

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

http://www.beeh.unp.ac.za/acru