diff --git a/climax/main/config.py b/climax/main/config.py
index 7e868903b75aa9112a947a9f20b0748ef63afd9a..dcc14f2ae8406f9f88cdd63ca86100a3e194aed3 100644
--- a/climax/main/config.py
+++ b/climax/main/config.py
@@ -55,6 +55,11 @@ DEM_FEATURES = False
 # stratify training/validation set for precipitation by number of wet days
 STRATIFY = True
 
+# size of the validation set w.r.t. the training set
+# e.g., VALID_SIZE = 0.1 means: 90% of CALIB_PERIOD for training
+#                               10% of CALIB_PERIOD for validation
+VALID_SIZE = 0.1
+
 # whether to train using cross-validation
 CV = False