Therefore, the impact of TOU rates become apparent in years post 2010 in Figure 4. The time period highlighted in yellow, indicates the summer peak hours of 12:00PM to 6:00PM. In addition to TOU, several utilities offer various DR programs tailored toward agricultural irrigation customers with a combined load shed magnitude of 0.7 GW dating back to 2004. Although largely successful, challenges faced by agricultural DR programs include unreliable shed rates and low participation rates . Most agricultural irrigation systems operate in a manual or semi-automated fashion which require long notification periods in order to participate in DR programs. This along with challenges such as lack of communications, manual controls, and farm operational limitations has led to a low participation in DR programs by agricultural customers . Currently agricultural irrigation pumping can only participate in traditional DR programs offered through utilities . In the near future, fast responding DR services that can participate directly into the electricity markets will become more valuable . Automated DR , another DR strategy in which loads are shed automatically in response to grid control signals unless the customer opts-out, allows quicker, more reliable load shedding with less effort required by grid operators and growers alike. ADR has the potential to be used for ancillary services, which are growing in importance due to the load uncertainty and variability caused by the integration of large shares of renewables . Such services are referred to as supply side DR. In order to provide supply side DR to the grid, loads should directly interact with the California Independent System Operator . Besides limited pilot programs such as Demand Response Auction Mechanism , there are currently no other mechanisms in place that allows pumping loads to directly provide supply side DR,cultivo del arandano so agricultural customers can only provide resources to the grid by enrolling in a TOU, DR, or ADR program offered by their local utility or through a third party aggregator.The examples presented below illuminate the nature of the demand management challenges from the irrigators’ perspective.
Over-voltage incidents can result in significant damages to equipment and disrupt normal operations for extended periods. Therefore, demand management of agricultural loads is not only beneficial to the grid, but it also makes farming operations more resilient. In 2016, the peak demand of the California’s electricity grid was 46 GW . In the same year, the peak demand for agricultural irrigation pumping was 1.3 GW . As of 2015, California Investor Owned Utilities’ total DR portfolio was 2.1 GW . Theoretically, 62% of the current IOU DR portfolio can be satisfied through agricultural irrigation DR alone. Agricultural irrigation can help address several challenges highlighted in Figure 2. As shown in Figure 3, agricultural load is highly concentrated in the summer months, coincident with the peak demand of the grid as a whole. In addition, highest daily demand for agricultural irrigation occurs during hours with highest levels of evapotranspiration, which are coincident with highest levels of solar electricity generation. Solar curtailment, whereby solar generators are disconnected from the grid to protect the grid from being overwhelmed, occurs between the hours of 12-6 PM, hours of peak irrigation demand. A flexible and dynamic irrigation system can take excess load off the grid by over-irrigating during certain hours of the day in order to facilitate higher levels of solar integration into the grid and eliminate solar curtailment. In the absence of cost effective battery storage, irrigation pumping can be a valuable resource for balancing the electricity grid. Time of Use pricing is a cost effective option for modifying load shapes because there are minimal, if any, site-level technology enablement costs. And while the load reduction at any given site is typically small, the breadth of participation if the rates are default or mandatory provides a substantial statewide effect. TOU can contribute substantially to overall DR potential. The impacts of TOU pricing on agricultural accounts is clearly distinguishable in average daily demand profiles of agricultural accounts recorded by Pacific Gas and Electric’s Smart Meters as shown in Figure 4. Mandatory TOU rates were introduced in 2009 and over 75 percent of firms faced their first month of mandatory TOU pricing in the summer of 2010 .
This limited overview of demand management for irrigated agriculture in the San Joaquin Valley illustrates the management decisions that must be made. The following examples are based on an actual almond farm located in Turlock, California. The 92 acre farm is supplied by one groundwater pump. The farm received 38.4 inches of water, plus 4 inches of rainfall in 2017. In all the following examples, irrigation schedules are modified so that the water requirement of 38.4 inches is satisfied. The first case involves shifting time of use for a 92 acre almond orchard with ample delivery system capacity, a readily available water supply . The orchard is irrigated in three sets. Most irrigation events were 24 hours or more, so most irrigation events span three days. The actual sequence of irrigation dates and durations in 2017 is indicated by the histogram in Figure 5. The wide spacing between irrigation events indicates ample irrigation system capacity, allowing the farm to easily shift irrigation dates and durations. This represents an ideal opportunity for energy load shifting. It is simple to plan and implement, and presents a clear financial benefit. Energy rates for the farm are $0.195 per kWh for off peak hours and $0.445 per kWh for 8 peak hours daily. An alternative schedule, indicated in Figure 6, would restrict irrigations to the 16 off-peak hours each day. Capacity Bidding Program and Base Interruptible Program are examples of two DR incentive programs offered by Pacific Gas and Electric Company that are most suited for agricultural customers. The incentive program stipulates that interruptions will last no more than four hours, with no more than one interruption per day and no more than ten per month. In this example, we illustrate how the participation of the same farm as “Example 1” in a program similar to BIP2 will impact its normal operation. If the farm were also following the TOU schedule as illustrated in Figure 6, it will be operating close to maximum pumping capacity. The analysis begins with the irrigation schedule based on 16 hour sets presented in the previous example . A modified schedule with occasional interruptions generated at random times is overlaid on the TOU management-schedule . DR event days are illustrated with lighter bars. Irrigation events on DR event days do not exceed 12 hours to indicate a 4 hour interruption per event.
If an interruption is called when no pumping was planned it is indicated as a negative four-hour bar. On those days when no pumping was planned additional pumping for 8 to 12 hours can be inserted to compensate for preceding interruptions.It appears from Figure 7 that the irrigator could compensate for most interruptions shown by shifting irrigation dates by a day or two. The same total volume of water was applied in both Figure 6 and Figure 7. This example can also illustrate an important constraint common to DR programs, which is that a farm shall only be compensated for DR participation in months when they would normally be using a significant percentage of pumping capacity. For example, maceta hidroponica the requirement might stipulate that the pump enrolled in a DR program must operate at least 70% of the time. In this case the seasonal pumping with TOU considerations from May through August would exceed the 70% level. If the financial incentive for participating in the DR program were ~$8 per kW per month and the farm qualifies for four months, the payout would be an additional $2400 per year. However it is important to note that enrollment and successful participation in such a DR program could entail capital investment for remote system control and variable speed pumping, which are not considered here. Evapotranspiration3 is a widely used irrigation parameter for estimating crop yields and for estimating yield impacts when irrigation is limited . Depending on the crop, some degree of deficit irrigation may actually increase farm profits by reducing costs of water, energy and other inputs, and by increasing management flexibility. With some crops deficit irrigation can also improve crop quality if carefully applied at specific growth stages. Modeling of ET and the impacts of ET deficits during the season is, therefore, a central issue for DR management. Figure 8 shows a schedule for another orchard in which a similar TOU strategy as the first example was developed for a maximum of 15 off-peak hours per day. However, in this case the irrigation capacity could not meet scheduled crop water demands on six days in late July and August, indicated by red bars, each representing 15 hours of additional pumping needed to maintain the intended soil moisture pattern. The cumulative irrigation deficit during that interval would be 6% of intended seasonal water use. Scheduling of additional irrigations to compensate for the 6% deficit would involve significant rescheduling of water application to the field. And the farm orchard will not have an opportunity to catch up with lost irrigation until late August. Additional irrigations in late August will not mitigate the impacts to the crop of a month long period of stress from mid-July to mid-August. Because that deficit is concentrated in a one month interval and roughly coincides with the onset of harvest, effects on yields could be even more severe.The consequences of such periods of stress will depend on the complex relationships between irrigation timing and amounts, crop water availability, and crop response to available water. The ability of a crop to recover from a delayed or missed irrigation will depend on the stage of growth, the reserves of water in the soil, atmospheric conditions and the physiology of the crop.
Operating with crop stress as part of an irrigation strategy requires an advanced irrigation management model to estimate the effects of reduced crop water availability on the cumulative daily ET. Currently such advanced irrigation management models are not commercially available. As indicated in the previous sections, irrigation planning to accommodate TOU and DR strategies will need to anticipate occasions of high crop water demand weeks or months ahead of time, especially when allocating water among multiple fields that share a common water supply. If optimal water use involves some degree of deficit irrigation, the planner will need to assess the possible yield impacts incurred by delaying, reducing or eliminating some irrigations. This requires being able to estimate in advance and across the whole season the impact and value of each irrigation and how each irrigation will translate into crop available water at full or partial ET, particularly at critical growth stages. This requires sophisticated modeling of the relationships between the crop, the soils, the atmosphere and the irrigation system, combined with site specific measurements and the irrigator’s management goals. Meeting these challenges requires accurately modeling the disposition and fate of applied water and modeling crop response to available soil moisture not just daily, but looking forward over extended periods of time. Seasonal irrigation strategies and schedules need to be easily and quickly updated to match weather variations, the availability of water, disease problems and other factors that evolve during the season. And planning needs to account for farm-specific constraints due to contractual arrangements, operating practices, risk tolerance and other factors that differ from one farm to another. The most effective irrigation management technologies in the market today focus on monitoring daily and weekly estimated ET conditions to provide a limited water balance calculation. A water balance model calculates how much water is applied against ET estimates of how much water is used by the crop. While accurate on a weekly basis, these conventional methods of scientific irrigation scheduling do not provide adequate forecasting and accurate forward-looking schedules for the management challenges presented by deficit irrigation. Growers need to conduct long range planning and management of irrigation strategies, including deficit irrigation, to deal with these complex management challenges .
