fix tutorial for groundwater recharge calculation with EE

tutorials/groundwater-recharge-estimation/index.ipynb

@@ -1027,8 +1027,8 @@
       },
       "outputs": [],
       "source": [
-        "# Combine precipitation and evapotranspiration.\n",
-        "meteo = pr_m.combine(pet_m)\n",
+        "# Combine precipitation and evapotranspiration, and sort the collection.\n",
+        "meteo = pr_m.combine(pet_m).sort(\"system:time_start\")\n",
         "\n",
         "# Import meteorological data as an array at the location of interest.\n",
         "meteo_arr = meteo.getRegion(poi, scale).getInfo()\n",
@@ -1212,7 +1212,7 @@
         "***Case 2.2: the storage $ST_{m}$ is less than or equal to the water stored at the field capacity.***\n",
         "\n",
         "If $ST_{m} \u003c= ST_{FC}$, $APWL_{m}$ is implemented as follows:\n",
-        "$APWL_{m} = ST_{FC} \\times \\textrm{ln}(ST_{m}/ST_{FC})$, and no percolation occurs.\n",
+        "$APWL_{m} = - ST_{FC} \\times \\textrm{ln}(ST_{m}/ST_{FC})$, and no percolation occurs.\n",
         "\n",
         "#### Initialization\n",
         "\n",
@@ -1381,7 +1381,7 @@
         "    new_apwl = new_apwl.where(zone1, zone1_apwl)\n",
         "\n",
         "    # Calculate ST in zone 1.\n",
-        "    zone1_st = prev_st.multiply(\n",
+        "    zone1_st = stfc.multiply(\n",
         "        ee.Image.exp(zone1_apwl.divide(stfc).multiply(-1))\n",
         "    ).rename(\"st\")\n",
         "    # Implement ST in zone 1.\n",