3.3. Comparison of Modelled Peak Rates with Observed Datasets
Farm-level abstraction data: A comparison of the IWR-modelled peak monthly- and peak daily-to-annual rates against the observed data obtained from the 12 case study farms is given in
Table 5. There is reasonable agreement between the modelled and observed peak monthly-to-annual ratios. However, the IWR model considers only single-irrigated cropping systems. Multiple-irrigated cropping, as practiced on some of these commercial farms, would result in an extended irrigation season, with a consequent likely decrease in the peak monthly demand. However, the mix of high-value, short-season crops increasingly being grown within more traditional arable crop rotations reflects a greater range in peak monthly abstraction rates. There is also good agreement between the modelled and observed peak daily-to-annual ratios, although again, the observed range in actual abstraction rates shows greater variation than that within the modelled results. This is a reflection of different farming systems (with different crop mixes and irrigation schedules) being included in the 12 case study farms leading to a greater range in the observed peak monthly-to-annual ratios, compared to the modelling approach, which used a single crop and defined irrigation schedule. Overall, the comparison between the IWR-modelled data with the observed farm data confirms a high degree of confidence and a reasonably sound basis for developing guidelines for setting peak monthly and daily abstraction rates.
National abstraction data: The monthly pattern of irrigation abstraction in the EA Anglian Region is shown in
Figure 5, expressed as the percentage of the mean monthly abstracted volume to the mean annual abstracted volume. Over the period studied, July, on average, represents the peak month for abstraction, accounting for approximately 22% of total annual abstraction. The mean monthly variation between years is also shown. The timing of abstraction demand for the top 10% of licensed abstractors in the region for the summer period (April to September) is shown in
Figure 6, expressed as the ratio of mean monthly abstraction to total summer abstraction. The mean monthly variation between years is also shown. The data confirm that, on average, between 30% and 40% of total summer abstraction occurs in July, the peak month. However, on individual farms, this proportion was often much higher, perhaps, for example, where an abstraction licence was being used to irrigate specialist crops.
The variation between approximately 250 farms in the proportion of the licensed volume abstracted in the peak month is shown in
Figure 7. This shows that setting a 50% peak monthly-to-annual ratio (as suggested in
Section 3.1) would have satisfied 95% of irrigation abstractors’ needs. An analysis of the frequency distribution of these irrigated holdings confirmed that those whose peak monthly demand exceeded 50% accounted for only 6% of total abstraction in that year. These are likely to be small area farms, with small licenses, but irrigating high-value, shallow-rooting crops.
3.4. Other Considerations
The methodology described thus far relates to setting peak monthly and daily abstraction rates. There are, however, additional water resource and application equipment issues that must be taken into consideration in developing a scientifically robust methodology for use in setting peak abstraction rates within water permits or licenses.
Peak hourly and absolute rates: Once a peak daily rate has been determined, the peak hourly rate is largely set by the hours available to complete the irrigation and the flow requirements of the irrigation equipment. For all irrigation systems, the peak absolute and peak hourly rates can be the same; every system would be expected to run continuously for over an hour at peak.
Hours available: In the absence of abstraction restrictions, for example, when irrigating from a storage reservoir, most large-scale farming enterprises using overhead systems (e.g., sprinklers, hose-reel irrigators, or booms) would typically plan to complete their irrigation in about 16 h. This provides sufficient flexibility for unavoidable down-time, equipment maintenance, and moving equipment between fields, as well as reducing the need for equipment moves at night. There are, however, other circumstances that can also reduce the irrigation hours available. These could include, for example, (i) non-standard field sizes that make it impossible to use travelling irrigators for the full run-time every day; (ii) small-scale irrigators that may not have the staff and/or automation required to spread irrigation out over so many hours; (iii) lower evaporation, less wind, and off-peak electricity tariffs make it more efficient to irrigate only at night; (iv) golf courses sometimes need to irrigate in as few as 8 h overnight, starting after the last players leave at dusk and finishing in time for water to infiltrate before the early morning golfers arrive; (v) noise issues (diesel pumps, impact sprinklers) may restrict night-time irrigation, and (vi) health and safety considerations advise against moving portable equipment in the dark. In such cases, irrigators have to invest in additional irrigation equipment and apply the scheduled water in fewer hours, and, hence, will require abstraction licences with appropriate conditions and/or storage tanks to accommodate for localised peaks in demand.
Equipment flow rates: The flow rate of an irrigation system is set by its design. Most irrigation equipment has a wide range of flow rates available but can only be used over a narrow range once purchased. Hose-reel systems have a range of sizes and nozzles, but these must be matched to the size and shape of the fields as well as to the licence. Static sprinkler systems similarly need flexibility to be assembled in different arrangements on different fields. Minimum flow rates, to attain adequate uniformity, may require higher flows than typical working hours would suggest. Centre pivots and linear move systems often have a required flow rate set by the standard sprinkler packages available. Trickle (or drip irrigation) systems on row crops will typically apply the daily requirements in only one or two hours; however, the irrigation is cycled around sectors to minimise peak flows and pump size. Field size and shape may also influence the sector size.
Tanks and balancing reservoirs: Where abstraction is via a storage tank or reservoir, equipment flow rates are not an issue; the tank can be filled at a constant rate to balance higher short-term application rates. The use of a tank or balancing reservoir, therefore, allows an abstractor to smooth out peaks in irrigation demand and allows significantly lower peak ratios. Relatively small tanks can spread hourly peaks over a day’s abstraction; many days’ storage would be needed to spread daily peaks; a half season’s reservoir storage may be needed to spread monthly peaks. Where the irrigator also has a licence to abstract water during the winter months (high river flows), the storage reservoir may provide a balancing function for the summer abstraction by pumping via the reservoir. However, the additional capital cost and energy use due to double pumping does need to be considered.
Environmental sensitivity and flow constraints: The environmental sensitivity to peak abstraction rates depends on the water source. Minor streams and rivers whose flow is small compared to the abstractions are particularly sensitive to the peak absolute and peak hourly rates. Large surface water bodies, whose water level is barely affected by short-term pumping, are mostly sensitive to monthly or even total seasonal abstraction. Groundwater in unconfined aquifers is typically most sensitive to monthly or total seasonal abstraction; in confined aquifers shorter duration rates may be more important. The situation becomes more complex when one source is feeding into another, for example aquifers feeding spring lines or streams. In each case, it is recommended that the most sensitive peak rates be identified and set first. Sensitivity must also be judged in the context of the other licence conditions, and a regulatory requirement to balance not only the environmental impacts but also the economic implications of setting unnecessarily stringent constraints on peak rates of abstraction, which would have negative impacts on the financial value and benefits of water used for agriculture. Setting peak rates will also need to consider the future impacts of a changing climate with increased drought risks impacting on both agronomic volumetric demands and timing of irrigation, and any changes in the availability of water due to reduced river flows (). The number of different rates stipulated in a licence should, therefore, be kept to the minimum and only set where necessary. In catchments where both irrigation and aquatic dependent ecosystems are concentrated, environmental impact assessments may also be required to ensure environmental flows are not impacted by over-abstraction, particularly in dry years. However, most licences now have ‘Hands-off Flow’ (HoF) conditions which preclude farmers from abstracting once prescribed river flows (or groundwater levels) drop below defined levels to protect the environment and other abstractors.
Finally, it is worth mentioning that there is, unfortunately, no comparative scientific literature on the topic of peak rates for irrigation abstraction against which our modelled outputs can be compared. Further studies should be undertaken in other countries under differing crop and agroclimatic conditions to facilitate future comparison.