diff --git a/src/nhflotools/polder.py b/src/nhflotools/polder.py
index 2f1a77c..d017bba 100644
--- a/src/nhflotools/polder.py
+++ b/src/nhflotools/polder.py
@@ -1,3 +1,6 @@
+import itertools
+from collections import Counter, defaultdict
+
 import nlmod
 import numpy as np
 import scipy.interpolate as si