Predicting and mapping the future demand for
irrigation water in England and Wales
E.K. Weatherhead, J.W. Knox
*Natural Resources Management Department, School of Agriculture Food and Environment, Cranfield University, Silsoe, Bedford MK45 4DT, UK
Accepted 2 April 1999
Abstract
A methodology has been developed for predicting the future growth in demand for irrigation in countries with supplemental irrigation such as England and Wales. This takes into account expected changes in agricultural policy, technical, market and other factors. The methodology has also been applied within a geographical information system (GIS) to map the growth. The GIS approach represents an extension of a procedure developed by Knox et al. (1997) (Knox, J.W., Weatherhead, E.K., Bradley, R.I., 1997. Agricultural Water Management 33, 1±18).
The total net volumetric actual irrigation water requirements for a `design' dry year (20% exceedance), are predicted to rise from 168106m3in 1996 to 244106m3in 2021. Meanwhile,
the total net volumetric theoretical irrigation water requirements for a `design' dry year, are predicted to rise from 204106m3in 1996 to 247106m3in 2021.
Maps are presented showing the predicted change in the spatial distribution of irrigation demand between 1996 and 2021, and the theoretical dry year irrigation demand for all irrigated crops in 2021. The application and limitations of the methodology are discussed.#2000 Elsevier Science B.V. All rights reserved.
Keywords: England and Wales; GIS; Irrigation; Water demand; Maps; Prediction
1. Introduction
Recent droughts, rising public demand for mains water supply, and increased environ-mental protection, have all reduced the availability and reliability of water supplies for agricultural and horticultural irrigation in England and Wales. In many catchments, water
* Corresponding author. Tel.: +44-1525-863328; fax: +44-1525-863000. E-mail address: j.knox@cranfield.ac.uk (J.W. Knox).
resources are already over-committed, and additional licences for irrigation abstraction are unobtainable. Yet demand for irrigation water is also increasing (Weatherhead et al., 1997). Estimates of both the magnitude and location of that growth are an essential requirement for strategic water resource planning at national and regional levels.
Abstraction of water for spray irrigation in England and Wales is regulated by the Environment Agency (EA). The volume of water licensed and actually abstracted are reported annually. Both figures have increased substantially over the last 25 years (Fig. 1). The Ministry of Agriculture, Fisheries and Food (MAFF) also publishes statistics on agricultural irrigation in England and Wales, based on surveys carried out roughly triennially. These surveys report the areas irrigated and water use, by crop category, and in total. The areas that farmers reported they were `likely to irrigate in a dry year' from 1955 to 1995 (the most recent survey) are shown in Fig. 2. Rapid growth occurred up to 1965, followed by a decline until 1974. Since then the area has again risen sharply, despite a slight decline in 1995.
In 1980, a major national study by the Advisory Council for Agriculture and Horticulture (ACAH, 1980) predicted a 150% (4% per annum) growth in the irrigated area in England and Wales between 1977 and 2000, and an overall increase in water use of 300% (6% per annum), assuming no restrictions on water availability. However, the subsequent downturn in the profitability of irrigating grass kept actual growth well below the projected levels. There have also been many attempts to predict irrigation growth regionally, notably for Anglian region, where most irrigation occurs (e.g. Roughton and Clarke, 1978; Anglian Water Authority, 1982; Anglian Water Authority, 1988; National Rivers Authority, 1990). The predictions are summarised in Fig. 3. Although a variety of methodologies were adopted, common problems have been the difficulty in extricating the effects of recent weather, incorporating changes in external economic factors, and
Fig. 2. Total area `likely to be irrigated in a dry year' in England and Wales, 1955±1995.
Fig. 3. Selected predictions of future demand for irrigation in Anglian region, together with reported actual abstractions (1976±97).
predicting the effects of non-availability of water. Several studies failed to differentiate between the different irrigated crops; others failed to set sensible limits to the total crop areas or to the percentages of each that would be irrigated. Many of these studies were commissioned following periods of drought when interest in irrigation was abnormally high, and consequently over-estimated the subsequent growth in water demand.
Furthermore, all these studies used data aggregated over large areas, and ignored variations in demand due to the spatial variation in land use, soils and agroclimate. The development and application of GIS modelling techniques can now provide an effective tool for including such spatial variation (e.g. Madsen and Holst, 1990; Jacucci et al., 1995; Knox and Weatherhead, 1999). This paper describes an improved methodology for predicting the future growth in demand for irrigation in England and Wales, and its application within a GIS framework for mapping that growth. Where appropriate datasets are available, the procedures described are equally applicable in other countries.
2. Definitions of dry year demand
There are various ways of defining irrigation `demand'. In this study, two definitions are considered, termedactualdemand andtheoreticaldemand. Actual demand is based on the gross depths farmers are applying, as reported in the EA water abstraction and MAFF Irrigation Survey data. It therefore reflects directly the irrigation practices that farmers find realistic, and includes the effects of equipment constraints, present water shortages, scheduling errors, and the farmers' scheduling assumptions on irrigation losses. In contrast, theoretical demand is based on the calculated agronomic water requirements of the crops that are irrigated, assuming they are correctly irrigated following typical scheduling recommendations.
All forecasts presented in this study are for a `design' dry year, defined for the UK as a year with an irrigation need with a 20% probability of exceedance. In this context, 1990 can be considered as approximating to a `design' dry year, whilst 1995 was marginally more extreme. It is emphasised that all the predictions are ofdemand; actual water use will be reduced by any increased restrictions on water availability for irrigation. All forecasts are for the eight crop categories defined in the MAFF Agricultural and Horticultural Cropping Census and MAFF Irrigation Surveys (MAFF, 1996; MAFF, 1997); namely early potatoes, maincrop potatoes, sugar beet, vegetables (grown in the open), soft fruit, orchard fruit, cereals, and grass. For calculating irrigation needs, carrots were used to represent vegetables, strawberries for small fruit, and mature apples for orchard fruit.
3. Methodology
3.1. Overview
In an area where irrigation is supplemental to rainfall, such as the UK, many crops are not irrigated, and even for the irrigated crop categories, not all farmers irrigate.
Furthermore, many farmers may apply less than agronomic demand, either because of equipment or water limitations, or as a deliberate policy to maximise profit. The methodology required for estimating growth is thus more complex than for an arid area, where demand is more clearly a function of agronomic demand.
The demand for irrigation water in the UK therefore depends on the area of each crop grown, the proportion of each crop that is irrigated, and the depth of water to be applied to each crop. Each of these factors in turn depends on agro-economic and technical conditions which will inevitably change, as well as the fundamental agronomic and agroclimatic conditions, which will themselves vary.
The methodology adopted involves three stages. Firstly, the existing dry year baseline situation and the underlying growth rates (excluding annual weather variation) in each of the above factors are determined. Secondly, the future agro-economic and technical conditions must be modelled, and their influence on the growth rates estimated. Finally, the two must be combined in a way that produces feasible predictions.
The methodology was first applied in a spreadsheet model to predict growth inactual
demand at national level from a 1990 baseline (Weatherhead et al., 1994). Predictions were also made at county and regional level, but adjusted to maintain consistency with the national predictions. The methodology has now been updated to a 1995 baseline, and also applied using a GIS procedure, developed from work by Knox et al. (1997), to map the future growth intheoreticaldemand.
3.2. Underlying growth rates
In the UK, the irrigated areas and the volumes of irrigation water applied each year vary considerably depending on the summer weather. The data published in the MAFF Irrigation Surveys therefore partly reflect the weather in each census year, and do not directly show the dry year demand in a particular year or indicate the underlying trends in dry year demand (Knox et al., 1996). However, Weatherhead et al. (1994) developed a method for analysing the MAFF Irrigation Survey data using calculated theoretical irrigation needs (depths) for each crop as the independent climate variable in a multiple linear regression analysis. The regression results show the underlying growth rates in the areas irrigated, in the proportion of each crop irrigated and in the depth applied. They also allow the area and volume figures for any year to be adjusted to simulate `design' dry year conditions occurring at that time.
Weatherhead et al. (1994) analysed the underlying growth rates across four MAFF Irrigation Surveys, for 1982, 1984, 1987 and 1990. This procedure has been repeated incorporating data from the MAFF 1992 and 1995 Irrigation Surveys, to update the results and improve the statistical validity.
3.3. Future changes
The Manchester University Agricultural Policy Model (Burton, 1992) was used to predict changes in crop areas, prices and yields, up to the year 2021. Modelling was based on three world agricultural policy scenarios, namely (i) continuation of the 1992 conditions without Common Agricultural Policy (CAP) reforms; (ii) complete
liberalisation and free trade; and (iii) reform of the CAP under a new General Agreement on Tariff and Trade (GATT). This third scenario was considered by the authors to be the `most likely', and is used in this study.
The Manchester model predictions of changes in crop areas are applied directly to the baseline 1995 crop areas to predict future dry year crop areas. The predicted changes in crop prices and crop yields are used to help estimate changes in the underlying growth rates for the proportions of the crop irrigated and the depths of volumes applied. Predicted changes in market forces, and irrigation technology, cost and effectiveness must also be taken into account. Applying the changes to the growth rates rather than directly to the values themselves reflects the inertia and committed investment existing in the system. This stage inevitably remains partly subjective.
The resulting future rates of change (Table 1) are then applied to the baseline data, to predict future irrigated proportions and irrigation depths. For declining irrigated fractions, an ordinary compound rate of decline was assumed. For example, if the initial percentage change per annum isÿ5%, then 5% of the remaining irrigated fraction is lost each year. The irrigated fraction will thus approach zero asymptotically. However, for increasing irrigated fractions, a compound rate of decline of the remainingunirrigatedfraction was assumed. This ensures that increasing fractions approach unity (i.e. 100%) asymptoti-cally, rather than continue to grow. Similarly, increasing application depths were calculated by assuming a compound decline in the difference between the initial depth and a maximum depth, arbitrarily set at double the initial depth (in practice this maximum ceiling value was never reached and therefore had minimal effect on the result). The predicted crop areas, predicted proportions irrigated and predicted application depths are then combined. Irrigated areas and volumetric demands are calculated for each crop category, and then totalled.
3.4. Predicting future actual dry year demand
Weatherhead et al. (1994) used the MAFF, 1992 Agricultural and Horticultural Cropping Census data as the baseline for predicting crop areas, and the MAFF, 1990 Table 1
Future rates of change in the fraction of each crop irrigated and depth of irrigation water applied, based on Weatherhead et al. (1994)
Crop category Initial % change per annum
Fraction of crop irrigated Depth of irrigation water applied
Early potatoes 2 1
Maincrop potatoes 4 1
Sugar beet 2 0
Irrigation Survey as the baseline for predicting future irrigated fractions and irrigation depths. Future rates of change in the proportions irrigated and depths applied were based on the underlying trends from 1982 to 1990, the predictions for the three policy scenarios modelled, and the authors assessments of technological changes. A meeting of leading irrigators, irrigation advisers and others was used to judge whether the rates were reasonable.
Irrigated areas and volumetric demands were calculated for each crop category, and in total, up to 2021. High, medium and low predictions were produced for each of the three world agricultural policy scenarios. The modelling was also carried out at county level, with the results then adjusted for consistency with national totals. The adjusted county results were then re-aggregated to Environment Agency (EA) Regions.
The analysis has been repeated using the most recent MAFF cropping and irrigation census data. The predicted changes in crop areas were updated from 1992 to a 1995 baseline, and applied to the MAFF 1995 Agricultural and Horticultural Cropping Census data to estimate future crop areas. The MAFF 1995 Irrigation Survey data on irrigated areas and volumes applied were adjusted to correspond to a `design' dry year occurring in 1995, using regression factors derived from the 1982±95 underlying growth analysis (Section 3.2). The fraction of each crop irrigated and average irrigation depth, were then derived for this 1995 `design' dry year. Future rates of change in the proportions irrigated and depths applied were based on the underlying trends from 1982 to 1995, the predictions for the reformed CAP scenario, and the authors assessments of technological changes.
3.5. Predicting and mapping future theoretical dry year demand
Knox et al. (1997) developed a GIS procedure for mapping thepresenttotal theoretical dry year volumetric demand. For each of the main crops currently irrigated, irrigation needs (depths) were calculated by soil water balance modelling based on current agronomic recommendations, and then correlated to national datasets on agroclimate, soils and irrigation practice using a GIS, to generate irrigation need maps. These maps take into account the spatial variation in local soil type and climate. By multiplying these maps with datasets on irrigated land use, irrigation demand maps were produced for each crop category and in total. This procedure has been extended to predict the future
theoretical dry year demand.
The computerised spatial datasets used in both these studies have been described by Knox et al. (1997), but are briefly reviewed here. National soils (dominant soil association and profile available water) and agroclimate (potential soil moisture deficit) datasets were extracted from Land IS, the Land Information System held by the Soil Survey and Land Research Centre (SSLRC) (Hallett et al., 1996). The resolution of these raster (gridded) datasets were 1 and 5 km, respectively. Information on land use was obtained from raster datasets, based on the MAFF, 1994 Agricultural and Horticultural Cropping Census for England and Wales, at 2 km resolution. Information on the proportion of each crop that is irrigated were derived from the MAFF, 1995 Irrigation Survey, at county level.
Future changes in crop area were derived from the Manchester University Agricultural Policy Model predictions for the reformed CAP scenario, but adjusted to a 1994 baseline. For each crop category and predicted year, the future crop areas were estimated by multiplying the MAFF reported 1994 crop area in each 2 km pixel by these crop area change predictions.
Future changes in the fraction of each crop that would be irrigated in each pixel were estimated using the reformed CAP rates of change, at county level (the highest resolution of data available) (Table 1). Applying these to the future crop area datasets produces a series of predicted irrigated area maps, for each crop category, for five year intervals for 2001±2021.
Part of the GIS procedure developed by Knox et al. (1997) included the production of theoretical irrigation need (depth) maps, for each crop category. For this study, it was assumed that these theoretical values would not change over time (although agronomic recommendations could change, of course). The possible impact of climate change is considered later. Using a GIS overlay procedure, each predicted irrigated area map was then combined with the corresponding theoretical irrigation need map, to produce a theoretical demand map, for each crop category. By summing the individual maps, the total theoretical demand for each predicted year was calculated. Comparing these total theoretical demand maps allowed the production of maps showing the spatial variation in the growth of theoretical irrigation demand between 1996 and 2021.
4. Results
4.1. Underlying growth
The underlying growth rates from 1982 to 1995 in the area irrigated, volume applied and average depth applied, for each crop category, as a percentage of the 1995 value, are shown in Table 2. The underlying growth rate in the total volume of irrigation water
Table 2
Underlying growth rates in the area irrigated, volume applied and average depth, 1982±95
Crop category % change per annum on 1995 value
Area irrigated Volume applied Average depth
Early potatoes 1 (0.31) 4 (0.54) 4 (0.58)
r2values are given in brackets.
applied, was 3% per annum. The results show that irrigation is increasingly concentrated on the more valuable crops, notably maincrop potatoes, small fruit and vegetables, and that those crops that are irrigated are being given more water. In contrast, a marked decline in the irrigation of grass and orchard fruit is apparent.
4.2. Future actual dry year demand
Weatherhead et al. (1994) predicted a `most likely' growth in total volumetric demand, for a `design' dry year, of 1.7% per annum from 1996 to 2001, and 1% per annum from 2001 to 2021. These values have since been used in national water resource planning (e.g. National Rivers Authority, 1994).
The updated predicted actual irrigation demand from 1996 to 2021, by crop category, are shown in Table 3. The total net volumetric actual irrigation water requirements for a `design' dry year are predicted to rise from 168106m3in 1996 to 244106m3in 2021. This represents a national predicted average growth rate in actual volumetric demand for a dry year of 2.5% from 1996 to 2001, which then declines gradually, with an average of 1.5% from 2001 to 2021.
4.3. Future theoretical dry year demand
The predicted theoretical irrigation demands from 1996 to 2021, by crop category, are shown in Table 4. The theoretical total volumetric irrigation water requirements for a `design' dry year are predicted to rise from 204106m3in 1994 to 247106m3in 2021, an increase of 21%. A map showing the predicted theoretical total net volumetric irrigation demand for a `design' dry year, for all irrigated crops in 2021, is shown in Fig. 4. The likely spatial distribution of the changes in theoretical irrigation demand between 1996 and 2021 is shown in Fig. 5. These maps confirm that theoretical irrigation demand, and growth, will continue to be strongly concentrated in Eastern England, notably around the Fens region, and in parts of North Norfolk and the Suffolk coast. Parts of Kent, Nottinghamshire and Shropshire also show large increases.
Table 3
Predictedactualdry year demands (103m3), by crop category, from 1996±2021
Crop category Predicted actual irrigation demand
1996 2001 2006 2011 2016 2021
Early potatoes 9080 10110 11110 12040 12940 13740
Maincrop potatoes 73160 83430 90270 94920 97960 99890
Sugar beet 21650 23740 25800 27820 29800 31750
Orchard fruit 1880 2080 2260 2400 2540 2670
Small fruit 4990 7190 8970 10670 12140 13440
Vegetables 27770 35320 41530 47570 53640 59730
Grass 10510 8300 6560 5170 4080 3240
Cereals 7730 6420 5350 4410 3600 2880
Other crops 11110 12220 13350 14490 15650 16830
Total 167860 188820 205180 219500 232350 244160
5. Discussion
5.1. Actual or theoretical demand
The use of two different definitions of irrigation demand reflects different possible uses for the results and gaps in the availability of GIS datasets.
The projections of actual demand are based on current irrigation practices and the volumes farmers currently abstract, and are therefore of direct interest to planners. The projection methodology allows for predicted changes in irrigation practices, for example, due to increased investment in equipment, technology change, higher economic incentives, and improving irrigation efficiency. These projections are probably reasonably accurate reflections of the volumes farmers really want in the short to medium term, since such changes are likely to be slow, but they could be unreliable in the longer term.
The estimatedtheoreticaldemands probably overstate demands, since not all farmers can physically follow the scheduling recommendations, and others may be deliberately practising partial irrigation. Conversely, however, there is no allowance for losses. It is tempting to think that theoretical demand is somehow a fixed optimum, but that is misleading. Changes in equipment and economics can and should alter the recommendations, and hence the theoretical irrigation need. In particular, the percentage of the crop irrigated is a function of economic benefits and sometimes water availability. A GIS based approach to modelling demand allows consideration of local variability in cropping, soils and climate, and hence the production of irrigation demand maps. However, high quality spatial datasets are a pre-requisite for any GIS modelling. The base data for predictingactualdemand is only available at county level; this relatively coarse resolution would limit the accuracy and benefit of predicting actual demands within a GIS framework. In contrast, most of the datasets required to modeltheoreticaldemand are now available at a reasonable resolution, and GIS techniques are readily applicable. Using theoretical demand to study variability and comparing with actual demand at county level gives a useful compromise.
Table 4
Predictedtheoreticaldry year demands (103m3), by crop category, from 1996±2021
Crop category Predicted theoretical irrigation demands
1996 2001 2006 2011 2016 2021
Early potatoes 3290 3510 3710 3910 4050 4200
Maincrop potatoes 100290 109490 114140 115020 114500 112630
Sugar beet 26590 29110 31800 33950 36400 38930
Orchard fruit 4360 4420 4410 4420 4600 4560
Small fruit 6840 9350 10920 12180 12980 13570
Vegetables 37380 43730 47630 51240 54900 58100
Grass 15670 12700 10280 8100 5480 4570
Cereals 1340 1100 930 780 620 480
Other cropsa 7830 8540 8950 9180 9340 9480
Total 203590 221940 232760 238770 242860 246520
aEstimated.
5.2. Comparison with previous estimates
This study predicted a higher national growth rate in actual volumetric demand for a `design' dry year than previous estimates by Weatherhead et al. (1994). However, the differences in the predicted absolute volumes are not large, because of the revised baseline. Fig. 6 compares the predictions of total actual dry year demand arising from this study based on (a) the methodology described, (b) the same methodology but using the underlying trends from 1982 to 1995 without adjustment, and (c) the previous `most likely' predictions by Weatherhead et al. (1994). The predicted theoretical demand estimated in this study is also shown (d). The predicted total actual demands are Fig. 4. Predicted total net theoretical volumetric irrigation demand (m3kmÿ2) in a `design' dry year in 2021.
remarkably similar, although there are differences between crops and between areas, as discussed earlier. The actual demand predictions estimated in this study approach the theoretical demand prediction towards 2021, reflecting the increased depths applied on the dominant crops.
5.3. Regional and catchment predictions
By using the GIS to delineate administrative regions or hydrological catchments of interest, the local growth and future irrigation requirements can be estimated. This can also show the future split between crops in each sub-unit, and hence indicate the timing of Fig. 5. Predicted change in the spatial distribution of volumetric irrigation demand (m3kmÿ2) between 1996 and 2021.
demand and the likely take-up of particular water conservation measures such as trickle (drip) irrigation.
5.4. Influence of climate change
The Intergovernmental Panel on Climate Change (IPCC) reported that `the balance of evidence suggests there is a discernible human influence on the global climate' (Department of the Environment, 1996). Indeed the spate of recent droughts experienced in UK are consistent with a changing climate. However, the likely impacts on UK irrigation are still far from clear. Recent estimates suggest higher temperatures with only marginally more summer rainfall in the main UK irrigation areas (Department of the Environment, 1996). Others show a marginal decrease in summer rainfall, further increasing potential soil moisture deficits (BHS, 1998). Extrapolating from recent Institute of Hydrology predictions and past quantitative relationships between climate variation and summer rainfall, Herrington (1996) estimated an additional 27.5% demand above current trends in EA Anglian Region by the year 2021. However, as he cautions, using relationships based on pastvariationto estimate the effects ofchangeis likely to give an underestimate. Once the likely effects of climate change are more widely accepted, farmers can be expected to want to increase system capacity and irrigate more of their crops.
For calculating theoretical demand, the potential soil moisture deficit (PSMD), crop adjusted, is used as the climatic indicator within the GIS model. Once reliable climate change predictions are available for UK, it will be feasible to predict future changes in PSMD, and hence predict the increased theoretical demand for the crops currently irrigated. However, it will be much more difficult to model the effects of climate change
on cropping pattern and on the economics of irrigating particular crops. Furthermore, potential reductions in water availability due to climate change may themselves affect the location of irrigated agriculture.
5.5. Limitations
A number of assumptions were made in this study, and their respective limitations on the accuracy of the predictions made must be recognised. The limited accuracy of the MAFF data and the spatial integrity of the datasets used in the GIS has been discussed previously by Knox et al. (1997). It should also be recognised that integrating models with GIS can introduce an additional problem, notably the perception that GIS can `generate information' (Grayson et al., 1993). The sophisticated data handling and visualisation features of GIS can inadvertently seduce the user into an unrealistic sense of model accuracy, particularly where datasets with varying data structure (e.g. point, vector, raster), resolution, and format are incorporated. Output from GIS based analysis should therefore always be interpreted with caution.
The Manchester University model predictions appear to be reasonably robust with regard to the main irrigated crops, few of which are subject to subsidy under the CAP. The reformed CAP scenario still appears to be reasonable for the main irrigated crops. The subsequent choice of rates of change is partly subjective, and could be a major source of error. The short term predictions are based on recent trends and current expert opinion. In the medium to long term there is a risk of missing a new trend, for example, a change in the profitability of irrigating grass, cereals or sugar beet, a change to drought resistant varieties of potatoes, or the spread of new crops such as irrigated maize. Further research is needed to predict the effects of reductions in water availability and increased price on the economics of irrigation and hence on irrigated cropping patterns, and to predict whether and how restrictions on new abstraction licences will change the location of irrigated agriculture.
6. Conclusions
The spatial and temporal growth in the actual and theoretical dry year demand for irrigation in England and Wales has been predicted, incorporating forecasts of changes in the crop areas, in the fractions of each crop irrigated and in the irrigation application depths. The GIS procedure enables these predictions to be estimated and mapped by crop category, region, or catchment.
This study predicted higher underlying national growth rates than previous recent work, and greater future increases, although the differences in the predicted absolute volumes are not large, because of the revised baseline. The underlying volumetric growth from 1982 to 1995 was found to be 3% per annum. The total net volumetric actual
irrigation water requirements for a `design' dry year (20% exceedance), are predicted to rise by an average of 2.5% per annum from 1996 to 2001, and by an average of 1.5% per annum from 2001 to 2021. Meanwhile, the total net volumetric theoretical irrigation water requirements for a `design' dry year, are predicted to rise slightly less rapidly.
The results can help identify potential future water resource problem areas, and should be useful for water resource planning by individual farmers as well as by the government, regulatory authorities and others. The methodology can be applied in other countries where appropriate datasets are available.
Acknowledgements
This study formed part of a research contract (OC9219) funded by the Ministry of Agriculture, Fisheries and Food (MAFF). The authors acknowledge the support of the Soil Survey and Land Research Centre (SSLRC) for their support in digital data acquisition.
References
ACAH, 1980. Water for Irrigation: Future needs. Advisory Council for Agriculture and Horticulture in England and Wales. HMSO, London.
Anglian Water Authority, 1982. Anglian water forecasts of demand for direct water use and for in-river needs. Anglian Water Authority (AWA), Norwich.
Anglian Water Authority, 1988. Anglian water forecasts of demand for direct water use and for in-river needs. Operations Directorate (AWA), Norwich.
BHS, 1998. Climate change: water and environmental management in the Midlands. British Hydrological Society Circulation no. 57, 3±4.
Burton, M.P., 1992. An Agricultural Policy Model for the UK. Avebury.
Department of the Environment, 1996. Review of the Potential Effects of Climate Change in the United Kingdom. HMSO, London.
Grayson, R.B., Broschl, G., Barling, R.D., Moore, I.D., 1993. Process, scale and constraints to hydrological modelling in GIS. In: Kovar, K., Nachtnebel, H.P (Eds.), Application of Geographic Information Systems in Hydrology and Water Resources Management, Proceedings of the HydroGIS 93 Vienna Conference, Publication no. 211. IAHS, Wallingford, pp. 83±92.
Hallett, S.H., Jones, R.J.A., Keay, C.A., 1996. Environmental information systems developments for planning sustainable land use. Int. J. Geographical Information Systems 10, 47±64.
Herrington, P., 1996. Climate Change and the Demand for Water. Department of the Environment. HMSO, London. Jacucci, G., Kabat, P., Verrier, P.J., Teixeira, J.L., Steduto, P., Bertanzon, G., Giannerini, G., Huygen, J., Fernando, R.M., Hooijer, A.A., Simons, W., Toller, G., Tziallas, G., Uhrik, C., Yovchev, P., Van den Broek, B.J., 1995. HYDRA: a decision support model for irrigation water management. In: Pereira, L.S., van den Broek, B.J., Kabat, P., Allen, R.G. (Eds), Crop-water-simulation models in practice, selected papers of the 2nd Workshop, 15th Congress of the International Commission on Irrigation and Drainage, The Hague, Netherlands, 1993. Wageningen Press, Netherlands, pp. 315±32.
Knox, J.W., Weatherhead, E.K., Bradley, R.I., 1996. Mapping the spatial distribution of volumetric irrigation water requirements for maincrop potatoes in England and Wales. Agric. Water Manage. 31, 1±15. Knox, J.W., Weatherhead, E.K., Bradley, R.I., 1997. Mapping the total volumetric irrigation water requirements
in England and Wales. Agric. Water Manage. 33, 1±18.
Knox, J.W., Weatherhead, E.K., 1999. The application of GIS to irrigation water resource management in England and Wales. Geographical J. 165(1), 90±98.
Madsen, H.B., Holst, K.A., 1990. Mapping of irrigation need based on computerised soil and climatic data. Agric. Water Manage. 17, 391±407.
MAFF, 1996. Final Results of the June 1995 Agricultural and Horticultural Census: England and Wales, Regions and Counties. Ministry of Agriculture, Fisheries and Food, Guildford.
MAFF, 1997. Survey of Irrigation of Outdoor Crops in 1995. Ministry of Agriculture, Fisheries and Food, Guildford.
National Rivers Authority, 1990. Forecasts of abstraction demands for water, 1990 series, public water supply and direct abstraction. Anglian Region NRA, Peterborough.
National Rivers Authority, 1994. Water: Nature's Precious Resource. HMSO, London.
Roughton, J.L., Clarke, K.F., 1978. Evidence for the Advisory Council for Agriculture and Horticulture on the future water needs of the agriculture and horticulture industries. Anglian Water Authority, Norwich. Weatherhead, E.K., Place, A.J., Morris, J., Burton, M., 1994. Demand for Irrigation Water. NRA R and D Report
14, HMSO, London.
Weatherhead, E.K., Knox, J.W., Morris, J., Hess, T.M., Bradley, R.I., and Sanders, C.L., 1997. Irrigation Demand and On-Farm Water Conservation in England and Wales. Final Report to MAFF. Cranfield University, Bedford.
