Comparison of measured soil moisture deficits with estimates by MORECS

Transcription

1 The influence of man on the hydrological regime with special reference to representative and experimental basins L'influence de l'homme sur le régime hydrologique avec référence particulière aux études sur les bassins représentatifs et expérimentaux (Proceedings of the Helsinki Symposium, June 1980; Actes du Colloque d'helsinki, juin 1980): IAHS-AISH Publ. no Comparison of measured soil moisture deficits with estimates by MORECS C. M. K. GARDNER and J. P. BELL Institute of Hydrology, Wallingford, UK Abstract The validity of soil moisture deficits prepared by the British Meteorological Office Rainfall and Evaporation Calculation System, MORECS, is being investigated. This work is described and a sample or preliminary results is presented. Comparaison des résultats de mesure des besoins en eau des sols avec les estimations données par MORECS Résumé. On étudie la validité de calculs des besoins en eau des sols effectués par le système MORECS (Meteorological Office Rainfall and Evaporation Calculation System). On décrit ce travail et on présente quelques exemples de résultats préliminaires. INTRODUCTION The British Meteorological Office introduced a new system for estimating areal evaporation and soil moisture deficit in April 1978, known as MORECS (Meteorological Office Rainfall and Evaporation Calculation System). The system is designed as a service for the water industry and agriculture. It has great potential for short term water resource planning, so facilitating efficient water management. MORECS is much superior to the earlier Meteorological Office method for estimating areal evaporation and soil moisture deficit the so-caled E model (Grindley, 1967). The Institute of Hydrology is currently evaluating the accuracy of MORECS soil moisture deficit estimates. THE MORECS SYSTEM MORECS prepares weekly estimates of soil moisture deficit for the different types of land use within each square of a 40 x 40 km grid which covers the UK (Fig. 1). Mean values of soil moisture deficit for each square are also derived by weighting the soil moisture deficit data for the different land use types according to the land use distribution within the square. It is these composite values that are of use to the water engineer; the soil moisture deficit estimates for individual crop types are of more interest to agriculturalists. An account of MORECS is given by Wales-Smith and Arnott (1979) and Wales- Smith (1975) describes the early development of this system. Its principal features are as folows: (1) The calculation of average values of the Penman variables (air temperature, vapour pressure, wind speed and duration of strong sunshine) and rainfall for each grid square using daily data recorded at synoptic weather stations within each square. (2) The use of a modified Penman equation to calculate potential evaporation for each grid square. (3) The inclusion of a model to compute the evaporation of intercepted rainfall from the leaf canopy of different crops. (4) The incorporation of a simple two layer model to represent the extraction of soil moisture (i.e. actual evaporation) by different crops, and so calculate the soil 337

2 338 C. M. K. Gardner and J. P. Bel FIGURE 1. The MORECS grid and the distribution of the soil moisture measurement sites that are included in the databank. moisture deficit associated with each crop in a grid square. The basis of this model is the Penman root constant concept (Penman, 1949), using an exponential function to relate potential evaporation to actual evaporation at deficits greater than the 'root constant'. The model only accounts for inputs and outputs to the soil system via the soil surface; when the moisture content exceeds field capacity, the excess water is assumed to drain off the surface only (i.e. no drainage is aëowed for). The Meteorological Office have already made comparisons between some measured values and MORECS estimates of potential evaporation, actual evaporation and soil moisture deficit. They regarded the discrepancies as acceptable, allowing for the fact that areal MORECS estimates were compared with point measurements. However, these comparisons were restricted to grass, were very few in number, and were limited to a wet year, 1978 (Wales-Smith and Arnott, 1979). THE PROJECT The aim of the work at the Institute of Hydrology is to examine the accuracy of MORECS soil moisture deficit estimates more rigorously. Measured data for sites with a variety of crop and sou moisture conditions, occurring in as many grid squares as possible, are being compared with soil moisture deficit estimates for the appropriate crop type and grid square. This indicates the accuracy of MORECS predictions for individual crops in each of a number of basic soil types, and also the consequent likely errors in the estimation of the composite soil moisture deficit values. The model does not at present take account of soil type. By considering different crop/soil combinations it will also be possible to ascertain the extent to which the rainfaë interception and soil moisture extraction models distinguish between the behaviour of different crops, and the influence of soil type. It is expected that errors in the estimation of soil moisture deficit, which arise mainly in the estimation of actual evaporation from potential evaporation, will increase as the deficit increases. Figure 2 demonstrates the type of discrepancy that can thus occur in a dry year. Soil moisture deficit estimates for short rooted crops, prepared by

3 start of ^ measurements \\ M X s \ A. M. J, \ \ "\ ESTIMATED MEASURED - 2m profile 3.2m profile J v '\ A \. Comparison of measured with MORECS 339 S / J\,-*' i 0 Aw 7 N FIGURE 2. Estimates of soil moisture deficit for short rooted crops prepared by the E method compared with measurements at a grassland site. Field capacity was defined as the winter mean soil moisture content of the 2 m profile: a curve for the upper 3.2 m is included for interest. Note: soil moisture potential profiles were not available for 1976 but data from subsequent years demonstrate that the zero-flux plane was below the measured 3.2 m profile during this period. Therefore the measured deficit is entirely ascribable to evaporation and hence directly comparable with meteorological predictions. D the original Meteorological Office model, E, are compared with measured deficits at a grassland site for the drought year 1976 (MORECS estimates were not available until 1978). Penman's root constant method is used in the E model to calculate actual evaporation from potential evaporation. Thus, in this respect there is a close resemblance between it and MORECS. The E model, using a root constant of 75 mm, grossly underestimated the deficit which exceeded 260 mm in that year. It is particularly important therefore to evaluate MORECS estimations of deficit in dry years. For the purpose of this project a databank has been compiled from soil moisture measurements made in past years by many organizations throughout the UK. All of these measurements have been made with the use of neutron probes and so although field practice has varied, it is possible to standardize the data. The distribution of the sites for which soil moisture data are available is shown in Fig. 1. Data for the drought years of 1975 and 1976 are included in the databank which at present covers the period from The Meteorological Office is preparing retrospective MORECS data for the years prior to EXAMPLES Two examples of comparison between MORECS estimates and soil moisture deficit are shown in Fig. 3(a) & 3(b) for permanent grass and arable sites, for 1978, in grid square 170. Both sites are at Bridgets Experimental Husbandry Farm, near Winchester, in south-central England; they are located on Upper Chalk. In 1978, spring barley undersown with grass was grown at the arable site. 'Field capacity' has been defined as the soil moisture content of the 2 m profiles on the date (17 May 1978) after which zero-flux planes developed, i.e. when the transpiration demand of the plants first caused an upward flux of moisture (Wellings and Bell, 1980). At both sites this soil moisture content approximated the winter mean values. It is apparent in Fig. 3(a) & 3(b) that in 1978 the dominant tendency was for MORECS to overestimate the soil moisture deficit at these sites, although at both there is a remarkably good coincidence in the timing of the beginning of the main period of deficit in May and its termination in December. After harvesting in August the measured soil moisture depletion under the arable site tended to parallel that of

4 340 C. M. K. Gardner and J. P. Bed mm F F M/, AX NI -^-A^. \~ ''^'^ / M, Vy^Nf w ' ^ MEASURED MORECS ESTIMATES \ ^- -' \ V ^-/ \ j j 150- ^J- X - A r% A v r^~, s, s, barley ^harvest 0 0 N N a,k/ \/ / i 0 s [y j ; FIGURE 3. Comparison of MORECS estimates of soil moisture deficit with measurements at (a) a grassland site and (b) a site growing spring barley undersown with grass. At both the measured soil deficits were integrated for the upper 2 m. the grassland. MORECS assumes that after harvest, arable land is ploughed and evaporation takes place from the bare soil surface. For many of the soil moisture measurement sites in the databank no objective definition of field capacity is possible for three reasons: (1) soil water potential data are rarely available to define the onset of zero-flux plane conditions, (2) for many arable sites there are often no winter data which in rain-free periods could provide an estimate of field capacity, (3) the data do not always include measurements to a sufficient depth. Because of these difficulties in determining field capacities for all sites by a standard soil moisture method, field capacity has been defined in this study as the profile soil moisture content on the date each spring after which MORECS indicates that a persistent deficit has developed. Several methods are being considered to compare the measured and estimated soil moisture deficits in statistical terms. These might include, for example: (1) the size of the maximum difference between them, (2) the size of the difference at the time of maximum deficit, (3) the mean difference, (4) the size of the mean difference in excess of a threshold below which differences might be regarded as unimportant. In assessing the accuracy of the composite deficit estimates which are needed by regional water authorities for regional water resource planning and operational decisions, allowance will have to be made for the fact that data intended to represent 40 x 40 km grid squares are being compared with point data. Statistical methods to do this realistically are being investigated. For the purpose of evaluating the suitability of MORECS soil moisture deficit estimates for use in agriculture, particularly in irrigation scheduling, direct comparison of field data with areal estimates is appropriate because the farmer is provided with grid square mean data which he has to apply on a field scale. The discrepancy between the real and MORECS deficits, on the date when MORECS infers that the deficit has exceeded that when irrigation is required (e.g. 15 to 30 mm for grass, 15 mm for maincrop potatoes) may be most important in this context. b

5 CONCLUSION Comparison of measured with MORECS 341 Sample comparisons between MORECS and measured soil moisture deficits have shown that differences occur. Ths model overestimated deficits in 1978 and there is some evidence that underestimation may occur in dry years. Definition of the accuracy of MORECS deficit estimates for as wide as possible range of crops, soils and summer deficits is therefore important if they are to be used satisfactorily in either water resource planning or agriculture. The project described will enable the accuracy of MORECS estimates to be assessed and indicate how the system might be improved. Conducting such an evaluation is not straightforward, for problems including the definition of field capacity and the comparison of areal and point data have to be overcome. The physical reality of the field capacity concept is dubious in that drainage rarely ceases at all points throughout the whole profile simultaneously. Indeed, very large drainage losses can continue through the summer (Cooper, 1979). The comparison will reveal however the extent to which this consideration is of practical importance. Acknowledgements. This work is funded by the Department of the Environment, UK. The authors are grateful to all whom have contributed to the soil moisture databank; without their cooperation this project would not have been possible. REFERENCES Cooper, J. D. (In press) Measurement of moisture fluxes in unsaturated soil in Thetford Forest. Report no. 66, Institute of Hydrology, Wallingford, UK. Grindley, J. (1967) The estimation of soil moisture deficits. Met. Mag. Land., 96 (1137), Penman, H. L. (1949) The dependence of transpiration on weather and soil conditions. /. Soil ScL 1, Wales-Smith, B. G. (1975) The estimation of irrigation needs. In Engineering Hydrology Today, pp : Institution of Civil Engineers, London. Wales-Smith, B. G. and Arnott, J. A. (1979) The evaporation calculation system used in the United Kingdom. WMO Tech. Note. Wellings, S. R. and Bell, J. P. (1980) Water and nitrate fluxes in unsaturated Upper Chalk at Bridgets Experimental Husbandry Farm, Winchester. In The Influence of Man on the Hydrological Regime with Special Reference to Representative and Experimental Basins (Proceedings of the Helsinki Symposium), pp : IAHS Publ. no. 130.

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