2.1 Model formulation
In the present section, the required mathematical formulation of the model is presented. An objective function and a set of constraints are described below.
Objective functions
The problem of crop planning formulated for the study has two goals. At the same time, the model maximizes profit and minimizes groundwater use that irrigates crops. The goal is to use the limited resources effectively to determine the allocation of land for several crops to be planted annually. The objectives and constraints of the optimization model are:
Objective function 1:
Maximization of profit
Maximize Z1 =
[(Yi *Pi *Ai) - (Ci *Ai )] (1)
Objective function 2:
Minimization of irrigation water: The volume of groundwater to be used for irrigating the crops will be minimized due to the shortage of water in the study area.
$$\text{M}\text{i}\text{n}\text{i}\text{m}\text{i}\text{z}\text{e} {\text{Z}}_{2}={\sum }_{\text{i}=1}^{\text{n}}{(\text{W}}_{\text{r}\text{i}}\text{*}{\text{A}}_{\text{i}})$$
2
Constraints
i. Ground-water constraints: \(\sum {(\text{W}}_{\text{r}\text{i}}\text{*}{\text{A}}_{\text{i}})\le {\text{W}}_{\text{t}}\) (3)
The groundwater usage should be less than or equal to groundwater available for agriculture.
ii. Constraint regarding total available area for cultivation: \(\sum {\text{A}}_{\text{i}}\le {\text{A}}_{\text{t}}\) (4)
The sum of land area used for the cultivation of all crops in a given season must be less than or equal to the total available land area
iii. Non-negative constraints: \({\text{A}}_{\text{i}}\ge 0\) (5)
iv. Min-max area constraints: \({\text{L}}_{\text{b}\text{i}}\le {\text{A}}_{\text{i}}\le {\text{U}}_{\text{b}\text{i}}\) (6)
Crop area must fall within a given boundary.
Notations
i = 1, 2 … n
n = Number of crops considered in the study area; n= 14 (for kharif season) and n=11 (for rabi season)
\({\text{Z}}_{1}\) = Net profit (Rs)
\({\text{Z}}_{2}\) = groundwater use
\({\text{Y}}_{\text{i}}\) = Yield of ith crop per unit area (kg/ha)
\({\text{P}}_{\text{i}}\) = Price of each crop in the market (both main product and by-products) (Rs/kg)
$${\text{C}}_{\text{i}}={\left(\text{c}\text{o}\text{s}\text{t} \text{o}\text{f} \text{s}\text{e}\text{e}\text{d}\right)}_{\text{i}}+{\left(\text{c}\text{o}\text{s}\text{t} \text{o}\text{f} \text{h}\text{u}\text{m}\text{a}\text{n} \text{l}\text{a}\text{b}\text{o}\text{u}\text{r}\right)}_{\text{i}}+{\left(\text{c}\text{o}\text{s}\text{t} \text{o}\text{f} \text{m}\text{a}\text{c}\text{h}\text{i}\text{n}\text{e}\text{s}\right)}_{\text{i}}+{\left(\text{c}\text{o}\text{s}\text{t} \text{o}\text{f} \text{a}\text{n}\text{i}\text{m}\text{a}\text{l} \text{l}\text{a}\text{b}\text{o}\text{u}\text{r}\right)}_{\text{i}}+{\left(\text{c}\text{o}\text{s}\text{t} \text{o}\text{f} \text{f}\text{e}\text{r}\text{t}\text{i}\text{l}\text{i}\text{z}\text{e}\text{r}\text{s} \text{a}\text{n}\text{d} \text{m}\text{a}\text{n}\text{u}\text{r}\text{e}\text{s}\right)}_{\text{i}}+{\left(\text{m}\text{i}\text{s}\text{c}\text{e}\text{l}\text{l}\text{a}\text{n}\text{e}\text{o}\text{u}\text{s} \text{c}\text{o}\text{s}\text{t}\text{s}\right)}_{\text{i}}$$
\({\text{W}}_{\text{r}\text{i}}\) = Water requirement for each Crop (cubic meter)
\({\text{W}}_{\text{t}}\) = Total available water (cubic meter)
\({\text{A}}_{\text{i}}\) = Area under ith crop (thousand hectares)
\({\text{A}}_{\text{t}}\) = Total available area for cultivation (thousand hectares)
\({\text{L}}_{\text{b}\text{i}}\) = Lower bound of area for each crop (thousand hectares)
\({\text{U}}_{\text{b}\text{i}}\) = Upper bound of area for each crop (thousand hectares)
2.2 Non-Dominated Sorting Genetic Algorithm II (NSGA-II)
The NSGAII may be a second-generation MOEA developed by Deb et al. (2002) which improved upon the original NSGA by employing a more efficient non-domination sorting scheme, removing the sharing parameter, and adding an implicitly elitist method of selection that greatly helps to capture Pareto surfaces (Wang et al. 2011). In addition, the NSGA-II can handle both real and binary representations (Wei et al., 2009). NSGA-II was used in earlier crop planning studies successfully (Sarker and Ray 2009). The concept of Pareto-dominance is employed to assign fitness values to the sampling solutions. For instance, X1 dominates X2 if and only if it performs as well as X2 in all objectives and better in at least one objective. The fast-non-domination sorting approach of the NSGA-II ranks each solution consistent with the number of solutions that dominate it. Once fitness is assigned, a two-step crowded binary tournament selection is performed. In cases where two solutions have different ranks, the individual with the inferiority is preferred. Alternatively, if both solutions possess an equivalent rank, then the answer with the larger crowding distance is preferred. Solutions with higher crowding distances add more diversity to the answer population, which helps to make sure that the NSGA-II finds solutions along with the complete extent of the Pareto surface. A detailed step-by-step procedure is given in Figure 1 –
2.3 Non-dominated sorting algorithm-III
NSGA-II is extended and modified to NSGA-III (Deb & Jain, 2014; Jain & Deb, 2014) to improve the performance and efficiency of the algorithm. NSGA-II provides a set of Pareto optimal solutions which becomes problematic when the number of objective functions increases. Both NSGA-II and NSGA-II generate an initial random population of size N. In the case of NSGA-III, every solution represents a feasible scaling plan as described by Yannibelli et al. 2020. During the initial phase, both algorithms use similar kinds of evolutionary cycles such as the crossover and mutation operators. An offspring population is generated by NSGA-II using the same stage of mutation and crossover as NSGA-II. But in the case of selection mechanism, NSGA-III uses a different mechanism to create a new population for the next generation. A new population is created using for the successive generation from the combined current and offspring population. The termination criterion followed in NSGA-III is similar to NSGA-III. After achieving the termination criterion, NSGA-III generates a Pareto set of population corresponding to the final generation.