9.1: Introduction

When you consider installing an irrigation system, there are several water supply questions that must be answered. First, where is a suitable supply of water? More specifically, will the water come from a reservoir, a water course, or a well? Second, will water rights need to be addressed? The third question is about the quality of the water: is the water saline or will it be reclaimed water? Such questions are addressed in this chapter.

Although the most plentiful substance on the earth’s surface, water is frequently not available in sufficient quantity when and where it is needed. To overcome these deficiencies, water resources are frequently stored and then conveyed from the time and place of natural occurrence to the time and place of beneficial use. The demand for reliable water supplies continues to increase as world population grows and becomes more affluent. Depending on the location and climate, a large portion of water withdrawals are used for irrigation (Figure 9.1). Where the water supply is inadequate, competition arises between agricultural and urban water users and other users. In many locations, agricultural interests developed the water supply initially, thereby acquiring rights to the water through prior use. As urban water use increases, municipalities frequently can afford to pay higher water costs and can achieve a greater economic return per unit of water than agriculture. One solution to this dilemma is the purchase of agricultural land by municipalities to acquire the rights to the water. Another alternative is to seek rights to water through the legislative and judicial systems. Yet another potentially more attractive alternative is for the nonagricultural water users to pay for improved irrigation delivery or management systems with the water from reduced irrigation consumption going to the municipalities and agriculture being paid for the water (see also section 5.10). While this chapter focuses on water quantity for irrigation, the water quality of a water supply system should also be assessed for any potential negative impacts on the crop and soil (Chapter 7 and Suarez, 2012).

When considering the development of a water supply, water is categorized as surface water or groundwater. Surface water originates from precipitation on the landscape moving downslope to streams and rivers. A portion of the water in streams and rivers is from overland runoff. The balance is baseflow, streamflow that comes from groundwater. When flows are always ample to satisfy water demands, surface waters can be withdrawn directly from the natural water course. The flow of many water courses, however, fluctuates too widely over time to satisfy water demands. For many rivers, peak water demands occur at times of minimal flow. This situation requires the construction of reservoirs to store high flows to be released later for beneficial uses. Reservoirs are normally created by constructing a dam across a stream or river. In special situations reservoir sites are located off-stream. Surface storage may range in size from huge multipurpose reservoirs to small ponds.

Most precipitation that infiltrates and deep percolates beyond the plant root zone eventually reaches the groundwater table, called groundwater recharge. Groundwater is water beneath the earth’s surface that occurs in saturated materials. A zone of saturation in a substratum capable of yielding enough water to satisfy a particular demand is referred to as an aquifer. A major challenge facing water managers is ensuring that withdrawals of groundwater plus baseflow requirements do not exceed recharge. In many areas where groundwater is the major water supply, withdrawals exceed replenishment, and a sustainable water supply is in jeopardy.

Figure 9.1. Irrigation withdrawals by state for 2015 (top) and irrigation withdrawals over time (bottom). (Both illustrations from Dieter et al., 2018.)