Water Balance Module description

Soil Water (Moisture) balance equation at land parcel scale is defined as:

Change in soil moisture storage =Rainfall + Irrigation - Surface runoff - Evapotranspiration - Deep percolation.

Step By Step Process

• The web page will have the drop down for selecting District, Taluk, Village, Watershed name, Survey Number.
• User will select the District, Taluk, Village, Micro Watershed, Survey number.
• Calculate Surface runoff using DSS for Surface runoff
• Calculate ETc using DSS for Crop water Requirement.
• Subtract runoff from the rainfall obtained for that particular day.

                                               

                                  Where,

                                              ∆S = Water Balance

                                              P= Rainfall

                                              P_e= Surface Runoff 

• To initiate the water balance for the root zone, the initial depletion Dr-1 should be estimated. Calculation of initial depletion (Dr,i-1 ) is given as:

                                     

Where,

                      q_FC – Water content at field capacity [m³ m^-3] (from data)

                      Z_r – The rooting depth [m] (from data)

                      q_i-1 is the average soil water content for the effective root zone. Following heavy rain or irrigation, the user can assume that the root zone is near field  capacity, i.e.:                                                           

• Calculate the Readily Available soil Water (RAW):

                                                   RAW=pTAW

Where,

                      RAW - Readily available soil water in the root zone [mm],

p - Average fraction of Total Available Soil Water (TAW) that can be depleted      from the root zone before moisture stress (reduction in ET).

• Calculate the Total Available soil Water (TAW):

                                              

Where,

            TAW – Total available Soil water in the root zone [mm]

            q_FC – Water content at field capacity [m³ m^-3] (from data)

            q_WP – Water content at wilting point [m³ m^-3](from data)

            Z_r – The rooting depth [m] (from data)

• Excess balanced water beyond soil depth is assumed as Deep percolation. It is estimated as follows: 

                                        

Where,

            DP - water loss out of the root zone by deep percolation on day i [mm]

            P_i - rainfall on day i [mm]

            P_e,i - runoff from the soil surface on day i [mm]

            I_i - net irrigation depth on day i that infiltrates the soil [mm]

            ET_C,i - crop evapotranspiration on day i [mm]

            D_r,i-1 - water content in the root zone (D_r) at the end of the previous day, i-1 [mm]

• Irrigation is required when 

                                             

• Irrigation for the particular crop will be with reference to Package of Practice.
• Add Irrigation (I) value to the Soil Moisture storage()
• Subtract the Deep Percolation (DP) from Soil moisture storage ()
• If RAW is lesser than ETc, adjust crop water requirement (ET_{C adj}) based on moisture stress. The effects of soil water stress are described by multiplying the Crop coefficient (K_C) by the water stress coefficient(K_S):

                                                  

Where,

            ET_{C adj} –Adjusted crop water requirement

            K_C – Crop Coefficient

            K_S – Water stress coefficient

            ET_o – Potential evapotranspiration

• For Dr > RAW, Ks is given by:

                                                  

Where,

          K_S - Water stress coefficient

          D_r - root zone depletion [mm],

          TAW - total available soil water in the root zone [mm],

          RAW - Readily available soil water in the root zone [mm]

          p - Fraction of TAW that a crop can extract from the root zone without suffering   water stress.   

• When the root zone depletion (Dr) is smaller than RAW, Ks = 1
• Fraction of TAW, P will be taken from the data provided.
• Subtract ETC adj from soil moisture storage ().
• Display the result.