Technical Reference

This document describes the complete Ribasim - MetaSWAP - MODFLOW 6 coupling sequence, the operations performed by the individual components as well as the exchanges of data among the components.

The exchanges between kernels can be summarized as follows:

1. Exchange the Ribasim subgrid stage from the previous timestep to actively coupled RIV en DRN packages.
2. Estimated RIV and DRN flux, exchanged to Ribasim basins

Ribasim-MetaSWAP sub-timestepping:
3. Runoff from MetaSWAP exchanged to Ribasim basins
4. Irrigation demand from MetaSWAP svats to Ribasim water users.
5. Irrigation volume on svats based on realized volumes per water user
6. Flux correction in the case that Ribasim basins are unable to meet the total infiltration demand.

MODFLOW 6-MetaSWAP iterations:
7. Head of MODFLOW 6 to MetaSWAP
8. Storage coefficient from MetaSWAP sent to MODFLOW 6
9. Recharge from MetaSWAP sent to MODFLOW 6 as a source term
10. Groundwater sprinkling flux requested by MetaSWAP, extracted from MODFLOW 6

Some remarks

• If allocation is active in the Ribasim model, the demand-realisation cycle for irrigation is based on the user priority and the water availability. When inactive it is based on water availability alone.
  
• For coupling surface water to Ribasim, only the RIV and DRN package can be used. Other packages like GHB are not supported and should not be present in coupled domain.

Active versus passive coupling

In coupling Ribasim and MODFLO 6, a distinction exists between active and passive coupling. In a passive coupling fluxes on the MODFLOW 6 side are evaluated irrespective of the Ribasim waterlevels and contribute directly to the Ribasim basins. The Ribasim water levels are not required in this case, fluxes are simply regarded as “lateral” sources to the basin, hence configuration on the Ribasim side is minimal. The passive approach is justified for drainage packages showing little variation in terms of elevation over time (e.g. surface runoff, or ditches with negligible water depth). On the contrary, in the case of an active coupling fluxes depend (linearly) on the Ribasim water levels. This requires the stages in the MODFLOW 6 model to be set to the subgrid water levels of Ribasim prior to evaluation of the river fluxes.

In the case of river packages “over-infiltration” can cause Ribasim water levels can fall to zero, leaving a dry basin. Since in the passive situation, water levels are not taken into account, no feedback occurs that limits or impedes infiltration. The MODFLOW 6 model assumes infiltration to continue, whereas the coupled Ribasim basin already ran out of water. This raises a discrepancy in the water balance between both models. Therefore, passive couplings are unsuitable for river packages In active couplings, infiltration stops when the basin dries up and the water level reaches the bed elevation.

Estimated sprinkling flux

Within the Ribasim context, the sprinkling flux is implemented as a water user. The sprinkling demand is exchanged to Ribasim through the user demand matrix. From the couplers perspective, this is a two-dimensional array in which every column represents a water user and every row corresponds with a priority. The array elements are water demands for the users (also for other water users beside sprinkling), each with their own set of priorities. Priorities play a role on the Ribasim side only if allocation is active. Within Ribasim, water users are defined as nodes in the network and in the coupler water users are mapped onto MetaSWAP svats. For every of those users, the water demand can be non-zero for a single priority only.

Realized sprinkling flux and infiltration flux correction.

Before performing a Ribasim update, the surface water sprinkling amounts in MetaSWAP and the active infiltration or drainage in MODFLOW 6 have been evaluated under the assumption of an unlimited availability of surface water for these processes (steps 2 and 4) . After Ribasims timestep, the realized extraction is known in Ribasim and this information must somehow be communicated to MODFLOW 6 (step 6) and MetaSWAP (step 5) in order to maintain a correct overall water balance.

For the surface water sprinkling the following procedure is used to derive the realized fluxes for the MetaSWAP units.

• The realized fraction $f_i$ is derived of the realized sprinkling flux $R_i$, compared to the requested flux $D_i$ for Ribasim element $i$. i.o.w. $$f_i=\begin{array}{r}\{\begin{array}{ll}{R}_i/D_i& D_i\ne 0\\ 0& D_i=0\end{array}\end{array}$$

• The real contribution of Ribasim element $i$ to MetaSWAP element $j$ was then $f_i{A}_{ij}{M}_j^{\ast }$, where $M^{\ast }$ denotes the MetaSWAP vector of demand fluxes and $A_{ij}$ refers to an element of the couple matrix.

• So, the total realized flux $M_j$ received by MetaSWAP element $j$ is then obtained by summation over $j$: $$M_j=\sum _j{f}_i{A}_{ij}{M}_j^{\ast }$$,,, or in vector notation for all MetaSWAP elements: $$\mathbf{M}=(A^T\mathbf{f})\odot {\mathbf{M}}^{\mathbf{\ast }}$$

The evaluation of the correction of the infiltration flux, for MODFLOW 6 proceeds in a similar fashion, except:

• Not the realized fraction $f_i$, but its complement $1-f_i$ is used to obtain the correction $C_j$ on the infiltration flux.

• This correction is only applied to the negative demands. The positive demands, associated with drainage, are always granted.

• The correction amounts represent a lack of water already infiltrated into MODFLOW 6, which turned out not to be available at the end of the Ribasim timestep. Hence, this is corrected by means of a negative source term in MODFLOW 6 in the next groundwater timestep. $$\begin{array}{r}{C}_j=\{\begin{array}{ll}\sum _j(1-f_i)A_{ij}{M}_j^{\ast }& M_j^{\ast }<0\\ 0& M_j^{\ast }>0\end{array}\end{array}$$

This must be taken into account into the realized fraction for a correct water balance. Therefore, the demand $D_i$ to Ribasim element $i$ is now plit into $D_{i-}$ and, $D_{i+}$, summing fluxes over the infiltrating and draining MODFLOW 6 elements respectively.

$$f_i=\begin{array}{r}\{\begin{array}{ll}(R_i-D_{i+})/D_{i-}& D_{i-}\ne 0\\ 0& D_{i-}=0\end{array}\end{array}$$

The correction $C_j$ enters MODFLOW 6 as a negative right-hand side term, i.e. extraction.