--- a/artifacts/src/main/java/org/dive4elements/river/artifacts/model/WQDay.java	Fri Nov 01 12:12:24 2013 +0100
+++ b/artifacts/src/main/java/org/dive4elements/river/artifacts/model/WQDay.java	Fri Nov 01 13:25:54 2013 +0100
@@ -8,6 +8,12 @@
 
 package org.dive4elements.river.artifacts.model;
 
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+
+import org.dive4elements.river.artifacts.math.Linear;
+
 import gnu.trove.TIntArrayList;
 
 /**
@@ -19,6 +25,19 @@
 public class WQDay
 extends      WQ
 {
+    public static final Comparator<double []> FIRST_CMP = new Comparator<double []>() {
+        @Override
+        public int compare(double [] a, double [] b) {
+            double diff = a[0] - b[0];
+            if (diff < 0d) return -1;
+            if (diff > 0d) return +1;
+            return 0;
+        }
+    };
+
+    public static final double EPSILON = 1e-4;
+
+
     protected TIntArrayList days;
 
     public WQDay() {
@@ -52,6 +71,68 @@
         return days.getQuick(idx);
     }
 
+    private static final Double interpolateX(ArrayList<double []> dxs, double x) {
+
+        Collections.sort(dxs, FIRST_CMP);
+
+        if (Math.abs(x - dxs.get(0)[1]) < EPSILON) {
+            return dxs.get(0)[0];
+        }
+
+        for (int i = 1, S = dxs.size(); i < S; ++i) {
+            double [] curr = dxs.get(i);
+            if (Math.abs(x - curr[1]) < EPSILON) {
+                return curr[0];
+            }
+
+            double [] prev = dxs.get(i-1);
+            double x1 = Math.min(prev[1], curr[1]);
+            double x2 = Math.max(prev[1], curr[1]);
+            if (x > x1 && x < x2) {
+                return Linear.linear(
+                    x,
+                    prev[1], curr[1],
+                    prev[0], curr[0]);
+            }
+        }
+
+        return null;
+    }
+
+    public Double interpolateDayByW(double w) {
+
+        int S = days.size();
+
+        if (S == 0) {
+            return null;
+        }
+
+        ArrayList<double[]> dws = new ArrayList<double[]>(S);
+
+        for (int i = 0; i < S; ++i) {
+            dws.add(new double[] { getDay(i), getW(i) });
+        }
+
+        return interpolateX(dws, w);
+    }
+
+    public Double interpolateDayByQ(double q) {
+
+        int S = days.size();
+
+        if (S == 0) {
+            return null;
+        }
+
+        ArrayList<double[]> dqs = new ArrayList<double[]>(S);
+
+        for (int i = 0; i < S; ++i) {
+            dqs.add(new double[] { getDay(i), getQ(i) });
+        }
+
+        return interpolateX(dqs, q);
+    }
+
     @Override
     public void removeNaNs() {