Update water balance error definition (#1767)

core/src/write.jl

@@ -158,13 +158,9 @@ function basin_table(
     storage_rate = Δstorage ./ Δtime
 
     for i in 1:nrows
-        storage_flow = storage_rate[i]
-        storage_increase = max(storage_flow, 0.0)
-        storage_decrease = max(-storage_flow, 0.0)
-
-        total_in = inflow_rate[i] + precipitation[i] + drainage[i] - storage_increase
-        total_out = outflow_rate[i] + evaporation[i] + infiltration[i] - storage_decrease
-        balance_error[i] = total_in - total_out
+        total_in = inflow_rate[i] + precipitation[i] + drainage[i]
+        total_out = outflow_rate[i] + evaporation[i] + infiltration[i]
+        balance_error[i] = storage_rate[i] - (total_in - total_out)
         mean_flow_rate = 0.5 * (total_in + total_out)
         if mean_flow_rate != 0
             relative_error[i] = balance_error[i] / mean_flow_rate

docs/reference/usage.qmd

@@ -194,8 +194,10 @@ The Basin table contains:
   It will be placed in a separate output state file in the future.
 - The `inflow_rate` and `outflow_rate` are the sum of the flows from other nodes into and out of the Basin respectively.
   The actual flows determine in which term they are counted, not the edge direction.
-- The `storage_rate` is flow that adds to the storage in the Basin, increasing the water level. In the equations below this number is split out into two non-negative numbers, `storage_increase` and `storage_decrease`.
-- The `balance_error` is the difference of all Basin inflows (`total_inflow`) and outflows (`total_outflow`), that is (`inflow_rate` + `precipitation` + `drainage` - `storage_increase`) - (`outflow_rate` + `evaporation` + `infiltration` - `storage_decrease`).
+- The `storage_rate` is the net mean flow that is needed to achieve the storage change between timesteps.
+- The `inflow_rate` consists of the sum of all modelled flows into the basin: `inflow_rate` (horizontal flows into the basin, independent of edge direction) + `precipitation` + `drainage`.
+- The `outflow_rate` consists of the sum of all modelled flows out of the basin: `outflow_rate` (horizontal flows out of the basin, idependent of edge direction) + `evaporation` + `infiltration`.
+- The `balance_error` is the difference between the `storage_rate` on one side and the `inflow_rate` and `outflow_rate` on the other side: `storage_rate` - (`inflow_rate` - `outflow_rate`).
   It can be used to check if the numerical error when solving the water balance is sufficiently small.
 - The `relative_error` is the fraction of the `balance_error` over the mean of the `total_inflow` and `total_outflow`.