Index: wflow-py/wflow/wflow_lib.py
===================================================================
diff -u -r4efaa4c2e535431943e073dca2c8a31a334f8a77 -r05d6720142c37c4619ebe43ad183e6854db971da
--- wflow-py/wflow/wflow_lib.py	(.../wflow_lib.py)	(revision 4efaa4c2e535431943e073dca2c8a31a334f8a77)
+++ wflow-py/wflow/wflow_lib.py	(.../wflow_lib.py)	(revision 05d6720142c37c4619ebe43ad183e6854db971da)
@@ -112,8 +112,8 @@
     percfull = ((storage + oldstorage) * 0.5) / maxstorage
     # first determine minimum (environmental) flow using a simple sigmoid curve to scale for target level
     fac = sCurve(percfull, a=minimum_full_perc, c=30.0)
-    demandRelease = fac * demand * timestepsecs
-    storage = storage - demandRelease
+    demandRelease = min(fac * demand * timestepsecs, storage)
+    storage =storage - demandRelease
 
     # Re-determine percfull
     percfull = ((storage + oldstorage) * 0.5) / maxstorage