diff --git a/gusto/equations/shallow_water_equations.py b/gusto/equations/shallow_water_equations.py
index e075f25f2..7c67c219b 100644
--- a/gusto/equations/shallow_water_equations.py
+++ b/gusto/equations/shallow_water_equations.py
@@ -566,6 +566,10 @@ def schur_complement_form(self, alpha=0.5, tau_values=None):
             # bbar was be_bar and here we correct to become bbar
             bref += self.parameters.beta2 * self.qvbar
 
+            # however, we leave b_ as is, and do not add a contribution from qv.
+            # It already represents the perturbation to b_e, and since we
+            # neglect perturbations to qv, the equation below is already correct
+
         seqn = (
             inner(w_, (u_)) * dx
             - beta_u * (D_) * div(w_*bref) * dx