**1. Introduction**

Agriculture has a crucial part in human water resource use [1]. Approximately 70% out of the total available freshwater consumption is applied for water to

support the agricultural output of the world [2]. About 18% of worldwide farmland yet produces almost 40% of food [3, 4]. Water resources have an impact on the productivity of a variety of anthropogenic activities that sustain livelihood [5, 6]. Increased demand for agricultural supplies has put a strain on the world's freshwater resources in recent decades, leading to their unsustainable use in many cases. With over one-fourth of the world's land area experiencing acute water shortage, [7] approximately 35% of people globally live in and around water-deficient places [8], Overexploitation of water resources usually occurs at the price of economic progress, resulting in environmental damage [9]. Thus, water is a necessity of almost all processes of production and means of production which represent the life-sustaining liquid as fuel for production systems. Crop plants cannot survive without water and limited supply prove havoc on the production levels. Thus, a sustained water supply is imperative to guarantee a higher yield of crops along with other climatic, edaphic, and genetic factors.

The water requirement for different crops, rainfall frequency, intensity, and effectiveness along with moisture regimes of soil is showing the irrigation requirement of crops.

$$\mathbf{IR} = \mathbf{W}\mathbf{R} - (\mathbf{ER} + \mathbf{S})\mathbf{R}$$

Where IR stands for irrigation requirement, WR is water requirement, ER exhibits effective rainfall and S is soil moisture contribution. The factors affecting education requirement are given as single crop irrigation need, area of the crop, and farm level distribution of water losses, all the factors are expressed in cm/ha or cm, mm.

#### **1.1 Net irrigation requirement**

It is defined as the quantity of water required in the form of depth to bring the soil moisture to its field capacity level for the evapotranspiration demand of the crops. It is also defined as the differentiation between the field ability and moisture content of the soil before irrigation (**Table 1**).


#### **Table 1.**

*Crops production, requirement and shortfall analysis for yield gap mitigation through sustainable water resource use in Pakistan.*

## **1.2 Gross irrigation requirement**

Gross irrigation need is the term used to describe the overall amount of water used for irrigation. Net irrigation requirements, water application losses, and other losses are included. The approximate losses at different phases of crop development can be considered to find the gross irrigation need for farms.

#### **1.3 Irrigation frequency**

The time interval between the two successive irrigations' during crop periods is known as irrigation frequency. It is showing the total number of dry days between irrigations during dry throughout the crop period. It is based on the pace at which plants absorb water, the field capacity of the soil, and the soil moisture present in the root zone. As a result, it depends on the crop, soil, and environment. In general, irrigation should be applied when the effective root zone, where most of the roots are concentrated, is about 50% and not more than 60% depleted of the available moisture. The interval (days) between two irrigations at the time of maximum crop growth, or peak crop consumption, is the irrigation frequency to be employed when constructing irrigation systems.

#### **1.4 Irrigation period**

The number of days that can be allowed for applying one irrigation to a specific design area during the crop's peak consumption time is known as the irrigation period.

#### **1.5 Growth duration**

The time it takes for various crops to grow varies greatly. Seasonal crops are like sorghum, maize, groundnuts, pulses, etc. that can only grow for one growing season. Crops like cotton, red gram, chilies, etc., whose growth duration spans two seasons, are called two seasonal crops.

#### **1.6 Critical phenological stages of cereals sensitive to moisture variation**

Germination stage—is the emergence of radicals from seed. Tillering—the division for differentiation and development of tillers. Shoot elongation—the phonological stage standing for internodal expansion. Booting—swelling and development of grain holding structure or peduncle. Heading/inflorescence initiation—ear head emergence from the leaf sheath. Flowering—the appearance of flowers.

Grain development—grain formation from fertilization to maturity which is further classified.

into Milky stage—milk-type fluid development.

Dough stage—dough raw material development.

Ripe stage—fully mature embryo just before shattering.
