D-Risk rationale

Field

Although typically considered a ‘wet’ region for food cropping, UK production of high-value agricultural and horticultural crops, such as potatoes and field-vegetables, is not immune to drought. Supplemental irrigation is an essential component of production for many crops, used to buffer short-term periods of rainfall scarcity, and is a highly productive use of water for crops that are very sensitive to drought stress. In England, small depths of irrigation applied to high-value crops generate substantial financial benefits not only to individual farming enterprises, but also in terms of ‘added value’ to the supply chain and those businesses engaged in processing and distribution of fresh fruit and vegetables.

The UK has been affected by several severe drought episodes including 1975-76, 1988-1992, 1995-1997, 2003, 2004-2006 and most recently in 2010-2012. Whilst the 1975-76 drought is widely recognised as being the most severe, recent drought events have also caused major impacts to agribusinesses and the rural economy. For example, previous research estimated the financial impacts of restrictions on irrigated production in East Anglia to be £144 million, excluding downstream impacts on the distributional supply chain.

A recent study as part of NERC’s Drought and Water Scarcity programme showed that almost three-quarters of respondents considered droughts to be a very important risk to their irrigated business, and nearly half believed it was highly likely that drought impacts will become more serious in the future, partly due to proposed abstraction reform that will reduce their licensed headroom.

In my view, you cannot plan exactly the same area and hope that is going to rain and you get the full crop. You have got to reduce the economic risk to your business

Although the proposed abstraction reforms are yet to be implemented, the likely consequences raise fundamentally important questions regarding:

  • how changes in future licensed allocations might impact on growers’ ability to cope with future dry years?
  • how businesses can maximise productivity and economic output?
  • how best to plan strategically for changes in water allocation and reliability?

Working with the farming community, we have developed D-Risk, a web-based decision support tool to help agribusinesses evaluate the impacts of abstraction licence changes on their short-term irrigated cropping programmes, and functionality to inform longer-term strategic options for reconciling drought risk with water availability.

Using D-Risk

Field

This section provides a brief explanation of the D-Risk webtool, including the farm data needed and how to interpret the key outputs. The explanation is divided into three sections:

  1. Description of input data for D-Risk;
  2. Description of D-Risk analysis, and;
  3. Interpretation of D-Risk outputs

Data input

Field

Farm data

Data input to the D-Risk webtool starts with entering your farm postcode. This will be used to retrieve weather data for the area. D-Risk conforms to Cranfield University’s legal requirements for data protection and confidentiality - no farm or licence data are stored on our server.

Next, it is necessary to provide D-Risk with information regarding the irrigated crops and the soil types on the farm. This requires you to follow a simple process for each irrigated crop:

  1. Select an irrigated ‘Crop type’ from the dropdown menu
  2. Select the ‘Soil type’ from the dropdown menu
  3. Select the ‘Typical planting month’ from the dropdown menu
  4. Enter the ‘Irrigated hectares’ for this crop-soil combination
  5. Click the ‘Add’ button

The same crop can be added multiple times to allow for differences in soil type, crop variety or irrigation plan. A crop can be removed using the ‘delete’ button at the end of their row (cross shaped button).

D-Risk automatically estimates your ‘design’ dry year and 'average year' irrigation needs (depths applied, mm) for each crop, assuming the irrigation is scheduled to maximise both yield and quality.

If required, you can modify the estimated ‘design’ irrigation needs by a percentage factor to match actual application depths. The percentage factor can be modified in each crop’s row either by typing it in or by clicking on the up and down arrows that appear in the box when placing the cursor over it.

Finally, the volumetric irrigation demand (m3) for the farm site is calculated based on irrigated areas, irrigation need for each crop type and the D-Risk ‘design’ dry year data for your location.

Note: A ‘design’ dry year is defined as a year with an irrigation need with an 80% probability of non-exceedance or the 16th driest year in 20 years.

Abstraction licence and storage data

Individual abstraction licence data needed for the farm are: water source (surface water, groundwater); purpose (direct or storage); annual and daily licence limits; and the specified start and end month for each licence. These can be found in your EA abstraction licence.

Entering an abstraction licence data into D-Risk requires you to complete the following using drop-down menus or data input boxes:

  1. Select the appropriate ‘Water source’
  2. Select the ‘Licence purpose’
  3. Enter the ‘Annual licence volume’ in m3
  4. Enter the ‘Daily licence limit’ in m3
  5. Select the ‘Start month’
  6. Select ‘End month’
  7. Click on the ‘Add’ button

Licences can be removed using the delete button at the end of their row (cross shaped button).

The total combined usable storage capacity for all irrigation reservoirs available on the farm can be entered under ‘Total usable storage capacity (m3)’. If no data are entered in this box, D-Risk assumes there is no storage capacity available.

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Summary description of D-Risk analysis

Lettuces

D-Risk uses a simple water balance to calculate the potential soil moisture deficit (PSMD) using monthly rainfall (P) and reference evapotranspiration (ETo) data, and the maximum PSMD (PSMDmax) is identified for each year. Using the crop and soils data provided by the user and the annual PSMDmax, D-Risk then calculates the theoretical annual irrigation needs for each crop-soil combination using a set of equations derived from previous research. By combining data on irrigation needs (depths water applied, mm) with the irrigated areas (ha) data, D-Risk calculates total water demand for each year.

A monthly time-step water balance model is then used to assess how irrigation demands compare against the annual licensed abstraction volume for the farm. In D-Risk, it is assumed that licenses dependent on surface water are used before licensed groundwater sources. Similarly, direct abstraction is preferred before storage, leaving farm reservoirs as the last source of water. Licence use is only possible between the specified start and end months.

From the water balance modelling, annual irrigation deficits and licensed abstraction ‘headroom’ are calculated. An irrigation deficit is assumed to be any proportion of annual demand that was not met either due to annual or monthly licence limits and/or not being able to supply water from reservoirs. Licensed ‘headroom’ is defined as the proportion of licensed volume that is not abstracted in any given year. It is calculated from the sum of all available licences (both direct and storage).

Because droughts are both rare and always different, the small number of past ‘real’ drought events are insufficient to provide a realistic assessment of future drought risk. Consequently, the MaRIUS project used a very complex regional climate model to produce a dataset of 100 series of ‘synthetic’ weather that could have occurred in the past. The data have been checked for robustness against actual historical records. Each weather series, of 30 years length, is equally probable weather. The D-Risk tool uses this data in the method described above in two ways:

  • Firstly, we have assumed that the future probability or likelihood of a given level of irrigation need is based on all of these time series and years - this is equivalent to basing the probability of rolling a given number on a dice on 3000 (100 x 30) throws of the dice. We have termed this analysis ‘long term’ conditions;
  • Secondly, we have assumed that the future probability or likelihood of irrigation need is based on the years in a single time series - this is equivalent to basing the probability of rolling a given number on a dice on just 30 throws of the dice. In this analysis, some of these time series will be drier (or have a greater number of extremely dry years) than others. These ‘driest’ and ‘wettest’ time series then provide an uncertainty boundary (envelope) around the long term ‘average’.
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D-Risk outputs and interpretation

D-Risk results are presented in the form of probability distribution functions both in graphs and tables. Two key outputs of relevance to growers are produced:

1. Irrigation deficit (m3)

Irrigation deficit is defined as the total volumetric irrigation demand not supplied in a year due to either monthly and/or annual licence limits being reached and/or insufficient water stored in reservoirs (where relevant).

The figure to the right shows an example graphical output for irrigation deficit, showing the probability or likelihood of exceeding a certain level of irrigation deficit in any year. The solid black line represents the long term probability (from the 3000 years of synthetic weather). So, for example, there is a 30% probability that the irrigation deficit will exceed 30,000 m3. Conversely, a deficit of 30,000 m3 will be exceeded in 30% of years over the long term).

Picture 1

Additionally, the figure also contains a solid grey area that represents the uncertainty region associated with the long term results, with the dashed lines representing the minimum (from the ‘wettest’ 30 year weather series) and maximum (from the ‘driest’ weather series) boundaries of the region. This means that for a single probability value, the deficit can vary around the average and be anywhere between the lower and upper boundaries of the uncertainty region. In the figure below, for a 30% probability of exceedance, the irrigation deficit could be between 20,000 m3 and 55,000 m3 (dashed orange arrows) depending on whether the coming years are significantly wetter or drier than the long term average. Conversely, for example, following the dashed blue lines shows that the probability of exceeding a deficit of 25,000 m3 ranges between 25% (best case) and 48% (worst case), with a long term exceedance probability of 37%.

2. Headroom (%)

Headroom represents the volume of licensed water not used in a given year. Since annual licence limits are assumed to be constant, headroom is represented as a proportion (%) of total licensed volume. A headroom of 0% this means that the business has reached the licensed volume limit and cannot abstract any further water for irrigation or for refilling reservoirs.

The graph to the right can be read in the same way as described above.

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Picture 2