SUNDIAL: General Information

The results of simple experiments and systems studies have been used to develop process-based models of nitrogen turnover in the soil/crop system. SUNDIAL is one such model (Bradbury et al, 1993; Smith et al, 1996). It as a dynamic model with a weekly description of the effects of different weather patterns, soil and crop types on nitrogen turnover. SUNDIAL is designed to be used in a ‘carry-forward’ mode, with information from one years run providing inputs for the next. This allows it to investigate more complex systems involving rotations of crops and planning timescales of several years (modelling whole farm systems). SUNDIAL is also used in modelling nitrate leaching from whole catchments (catchment modelling).

Concentrations of nitrate in natural waters (rivers, lakes and underground aquifers) have increased substantially over the last 30 years, and agricultural land is recognised as a major source of nitrate in drinking water in the United Kingdom (Archer and Thompson, 1993). Throughout the world there is political and legislative pressure to reduce nitrate leaching from agricultural land to meet maximum permitted concentrations of nitrate in drinking water. The maximum concentration permitted by the 1980 EC Directive on the Quality of Water Intended for Human Consumption is 50 mg nitrate l^-1 (Council of European Communities, 1980). There is also concern over the amount of nitrate reaching rivers, lakes and seas, which can cause algal blooms and eutrophication. The implementation of the EC Drinking Water Directive requires nitrate losses associated with agricultural management to be predicted. The SUNDIAL model can be used to predict nitrate losses by simulating the dynamics of nitrogen turnover in the soil/crop system. SUNDIAL-FRS can be used by farmers and advisors to devise a management strategy to minimise nitrate losses for individual farms.

The Nitrate issue

Nitrogen fertiliser usually contains nitrogen in one of three forms - ammonium, nitrate or urea. Once added to the soil, soil enzymes and microbes rapidly convert any urea and ammonium to nitrate, particularly when the soil is moist and reasonably warm. Nitrate is also produced from the breakdown of soil organic matter and from any manure or plant residues.The nitrate ion (NO₃^-) is very soluble in water, and so is readily leached out of the soil by water draining through it. Nitrate is lost to surface waters by run-off, or through land drains and to groundwater by deep percolation. In the United Kingdom, nitrate leaching occurs mainly in the autumn and winter, starting when soils reach field capacity in the autumn and ending as they begin to dry in the spring.

Plants absorb the nitrogen they need as ammonium or nitrate, so much of the nitrogen in the soil that is available for crop uptake is therefore vulnerable to leaching. This is the root of the ‘nitrate problem’. The key to minimising nitrate leaching is to ensure that the supply of nitrogen to the growing crop closely matches its demand, so that once the crop has ceased to grow and absorb nitrate from the soil, little remains at risk of leaching.

The development of SUNDIAL

SUNDIAL started life as the Rothamsted Nitrogen Turnover Model, intended to help scientists interpret nitrogen measurements in field experiments (Bradbury et al, 1993). In particular it aimed to predict how much nitrogen was released from the soil, and when this nitrogen release occurred (Bradbury et al, 1991). This work was primarily funded by the Home-Grown Cereals Authority (HGCA). SUNDIAL was developed as a user-friendly PC-based version of the original model (Smith et al, 1996). It is designed to be used both in retrospect, for example to compute nitrate losses from actual fertiliser applications and weather data, and in predictive mode, using long-term weather records or specified weather scenarios.

The development of SUNDIAL into a nitrogen Fertiliser Recommendation System (SUNDIAL-FRS ) was funded primarily by Defra, in consultation with members of the Association of Independent Crop Consultants (AICC) and horticultural input from The Wye Campus of Imperial College London .

For details of the reference given above, and for other relevant references go to References and further reading.

Back to Sundial home page