Agricultural Irrigation – Water Use Demand
The largest user of water both in the United States and many areas of the world is agriculture. With regard to management of the resource, water use in this industry has sustainability issues based on waste and improper crop selection. These two conditions were allowed to survive as long as they have largely due to the perceived endless amount of water when compared to the actual demand. This changed over time as the demand increased and supplies diminished. The largest culprits in the inefficient use of water for irrigation purposes are in many cases system leaks, evaporation and ineffective applications. While the water wasting is a more obvious issue, poor choice of crops is not.
Consider irrigation water transmission practices in the southwestern United States. In many areas irrigation water is both delivered and applied to the crops via open aqueducts. At the onset of this practice, many of the aqueducts were earthen ditches or canals that conveyed water from the point of greater to lesser abundance. In this case, water was lost primarily due to evaporation and seepage and due to the seemingly endless supply of water these were collectively considered acceptable losses. As time moved on and the “endless” water supply began to appear more limited, other measures were taken to help minimize this loss. These measures included lining the bottom of the aqueducts and in some cases replacement with piping systems as well as considerations towards installing covers. In the past, as in the present, decisions to upgrade these conveyance systems had an economic hurdle. As water availability lessens leading to other sources being sought out at an elevated cost point, the economic pendulum swings towards improved management of transmission of this water supply to better control costs as well as maintain adequate and sustainable supplies.
Agricultural Irrigation – Site Conditions
Several conditions associated with ineffective irrigation are questionable volumes and/or the timing of applications based on actual need or demand. These can include all points from the source to application and can include overflowing open aqueducts and fields, application well beyond the instantaneous saturation point. Surcharging of the aqueducts and fields is wasteful and can lead to erosion in both cases and in the latter, releases of pesticides and fertilizer based nutrients that can lead to environmental issues elsewhere. While it is a good practice to apply adequate water to the root zone of the crop being irrigated, once you begin irrigating beyond that point is excessive and water tends to seep downward, continuing to travel further away from the root zone. Conversely if the application is not enough, the opportunity for salinization increases as do the chances of more of the root system remaining nearer to the surface subsequent to being more at risk during dry weather conditions. In consideration of daytime heating with respect to irrigation, application during the latter part of the day is preferred over late morning to late afternoon. While there will always evaporation based water loss, irrigation during cooler times of the day and lessens this effect.
Often times crop choices are based mainly on criteria related to growing temperatures as well as soil character. One factor not often given weighted consideration in the past was water utilization. Historically, if the crop of choice did well with hotter temperatures and the sandy loam soil found in an arid area with the only known deficiency being water, the crop was often times chosen without much further regard for the volume of water necessary. Some of the more water intensive crops are: sugarcane, rice, cotton, grapes, oranges, melons, potatoes, etc. Recently there has been a shift towards planting crops with water demands more consistent with their environment and in some cases restricting and/or discontinuing farming in an established area as dictated by reduced available water and the price point for the water that is available.
Salinization of soils has been a problem for a number of years, but has recently become more prominent in the United States Southwest as well as some of the coastal areas. With the irrigation water demand and water use dynamics detailed earlier, growers are using less water in part due to the volumes available as well as cost. About a year ago, farmers in the San Jose, California area were paying nearly $1,350 per acre-foot for water. Under non-drought conditions, the cost per acre-foot would have likely been less than $100. Shrinking surface water supplies will likely result in reduced quality, from both an increase in mineral concentrations as well as that of more general pollutants.
Reclaimed Water Use – Potable Supply
One of the actions taken to correct issues created by past water resource management practices, increasing demands and shrinking water supplies is greater utilization of the reclaimed water supply. Public perception of reclaimed water has often been that of a treated wastewater that still has a “yuck” factor associated with it. But, with increasing use led by increased demand, expanded use of reclaimed water is becoming more widely accepted. One of the largest and best examples of reclaimed water projects in the United States is the Water Conserv II project, a joint venture between Orange Countyand the City of Orlando, Floridawith distribution points located in west Orange County. From this site local growers, golf course owners, residents and the like receive highly treated water, capable of meeting most primary and secondary drinking water standards, for both irrigation and as well as freeze protection (this applies mostly to citrus crops) with the balance of the water being diverted to rapid infiltration basins (RIB’s) for groundwater recharge. The permitted public access irrigation is about 40 million gallons per day (MGD) while the RIB’s are permitted for about 30 MGD. This reclaimed water source has had a positive impact on the local agriculture and economy for both of the reasons cited above and is a good illustration of what can be done to minimize the effects of irrigation on groundwater supplies as well as the compounded effect of recharging aquifers to increase stores for future use.
Reclaimed Water – Increasing Role in Irrigation
The shrinking water supply issues are anticipated to increase in scale significantly before improving. The remedy will likely require a more holistic approach to strategic allocations of existing water sources for the most practical and beneficial end use. To that end, more water supplies will likely contain both potable and non-potable sources with better use of reclaimed water from the non-potable side taking on a more prominent role