Irrigation

WRSM-Pitman (Sami groundwater)

• Irrigation modules in WRSM cannot contribute return flows upstream of the source they are irrigated from. This means that areas irrigated by a farm dam are necessarily removed from its contributing catchment area even if they are actually located upstream of the dam in reality. This restriction is not present in other tools. If the relevant irrigated area is large in relation to the dam’s contributing catchment, this could lead to inaccurate dam inflow, particularly during large rain events producing surface flow.  With many smaller dams and hence associated smaller irrigation areas this becomes less of a problem than it would be if all small dams were lumped and had a large associated area not considered part of the contributing catchment.  
• Each irrigation area can only receive water from one source in WRSM, a single channel or reservoir module. A comparison of irrigation demand with potential surface water supplies over time was done to estimate the amount of irrigation that would have had to come from groundwater to maintain the observed agricultural production. In the model, groundwater withdrawal demand rates were input for the main runoff module of each quaternary/subcatchment to match the expected groundwater irrigation demand. The actual amount that would be withdrawn would be limited by the aquifer storage in the timestep. Groundwater withdrawals are removed from the model in WRSM, assumed to be used outside the catchment. To work around this, water was added to a ‘dummy channel’ module, as would be done for a flow transfer from outside the catchment. This amount was limited by the expected harvest potential of the aquifer. Specific groundwater irrigation areas were added to the model which were supplied by this channel module. The size of the groundwater irrigation areas was selected to match the estimated groundwater irrigation demand.
• A limitation of this approach is that the amount actually pumped from groundwater in the model and the amount of water made available to the groundwater irrigation areas are separate inputs. If the model groundwater storage is too low to allow groundwater pumping at a particular time, there should be no withdrawal, but this would not necessarily be included in the externally derived inflow time series. To prevent this an iterative approach is needed to check the withdrawals achieved compared to the inflows input. This process would also need to be redone if any relevant parameters or inputs are then changed (i.e. the irrigated area, crop type, rainfall, etc).  

SPATSIM-Pitman (Hughes groundwater)

• SPATSIM, as with WRSM, only allows irrigation from a single source for a given irrigation area in a subcatchment; includes irrigation from rivers, dams, and external input sources; and considers groundwater abstraction water to be removed from the model. As such a similar approach to WRSM would be needed for irrigation from groundwater, in which an irrigation area is specified to be irrigated from a water source external to the modelled catchment. This input is created to match the separately input groundwater abstraction. Small dams internal to a subcatchment are necessarily lumped into one unit per subcatchment in SPATSIM. The tool also includes a module for a reservoir at the downstream outlet of the subcatchment. This meant that two storage reservoirs can be included per subcatchment, as opposed to the many included in the WRSM set-up. This may result in surface water supply shortages being modelled at different times than were predicted in WRSM, in which specific areas would be limited by the amount available from a smaller local storage.    

ACRU4