From the weather forecast to the prognostic moisture content of dry agricultural crops

A.J. Atzema

Research output: Thesisinternal PhD, WU

Abstract

<em>Part 1</em><p>The aim of the study of grass is to forecast the drying of cut grass up to five days ahead, hourly. The first investigated problem is the response of the drying of cut grass to the weather elements. Next a simple model and an advanced model for the drying of cut grass are presented. Errors in the elements of the weather forecast which are important for the drying of grass, are discussed.<p>During three growing seasons the drying of grass in the field was observed from cutting to ensiling. The experiments were carried out on sixteen different plots at the same location. Every hour samples of grass were taken to determine the moisture content of grass. The grass species examined were diploid and tetraploid perennial ryegrass ( <em>Lolium perenne</em> L.) and diploid hybrid ryegrass ( <em>Lolium perenne x Lolium multiflorum</em> ) <em></em> var. Barcolte. The perennial ryegrasses were a mixture of different varieties. All the grasses examined were in the vegetative state. Also the cutting time and the swath treatments were examined.<p>Simultaneously with the samples of grass, meteorological data were gathered on the experimental plots: air temperature (1.5 m height), dew point temperature (1.5 m height), surface temperature of the grass, temperature within the grass swath and wind speed (2 m height). These meteorological data were compared with the standard meteorological data that were taken hourly from the nearest weather station.<p>Apart from the weather, the drying depended on the grass species and the yield. In general, the moisture content of grass at the time of cutting was 3% higher when cut in the morning than when cut in the late afternoon. Hybrid ryegrass had a lower initial moisture content at cutting and dried faster compared with perennial ryegrass. Diploid varieties dried faster than tetraploid at the same yields. Within the same varieties a low yield dried faster than a high yield. Frequent tedding did not influence the rate of drying significantly.<p>The temperature within the swath was comparable to the standard air temperature at 1.5 m. The standard air temperature at 0.1 m was a good indicator of the grass surface temperature. The grass surface temperature was mainly determined by global radiation and responded rapidly to its changes, but it was also influenced by the wind speed and the moisture content of the grass. A low moisture content of the grass allowed an increase of the grass surface temperature. Increasing wind speed had an adverse effect on the drying rate of grass.<p>The simple grass drying model uses no crop parameters and needs only three standard (forecast) weather elements: air temperature, dew point temperature and quantity of rain. In dry weather the moisture content of grass is calculated hourly during daytime from 10.00 h to 19.00 h (local time) by modelling the saturation deficit. Dew formed during the night is assumed to be compensated by the drying of grass early in the evening and in the morning before 10.00 h. Roughly this model gives an indication of the forecast moisture content of cut grass.<p>The advanced model uses some crop parameters and standard (forecast) weather elements. Crop parameters used are yield and the times of cutting, tedding and windrowing. Standard weather elements used are: air temperature, dew point temperature, global radiation, cloud cover, wind speed and quantity of rain. No differences in grass species are made through which the model can be used generally. When grass dries freely, absorbed water is lost by evaporation and transpiration. Condensation and precipitation increase the moisture content of grass. The formula of Penman-Monteith is used as basis for the calculation of the moisture content. In the model the grass swath is divided into two layers, and the upper layer is kept at a constant mass. So, during drying the upper layer grows in thickness by incorporating dry matter and water from the lower layer.<p>Favourable agreement was found between the advanced model calculation and the measured moisture content of grass in the field. Usually forecast weather elements do not have exactly the same value as the corresponding past weather elements. Errors in the forecast moisture content of grass are mostly caused by errors in the forecast air temperature, global radiation and precipitation. Errors in the forecast dew point temperature and wind speed play a minor role. Generally, the forecast of the moisture content of cut grass is significantly better for one day ahead than for more days ahead.<p><em>Part 2</em><p>The aim of the study of cereals is to forecast the moisture content of wheat and barley up to five days ahead, hourly. The first investigated problem is the relationship between standard weather data and the microclimate of wheat and barley during harvesting time. Next, a model for the calculation of the moisture content of wheat and barley is presented. Errors in the elements of the weather forecast which are important for the moisture content of cereals are discussed.<p>The moisture content of mature barley and wheat (from growth stadia DC 91) and the meteorological elements have been observed during three harvesting seasons. The experiments were carried out at three different sites in the Netherlands. Meteorological elements measured on the experimental plots were: air temperature, dew point temperature, surface temperature of the ears and global radiation. Air temperature and dew point temperature were measured at two heights: at the height of the ears and at 1.5 m height above the soil. The measurements were performed each hour from dawn till dusk. These local meteorological data were compared with standard meteorological data that were observed hourly at the nearest weather station. SUMMARY<p>Both in wheat and in barley no significant difference is present in the dew point temperature (1.5 in height) between the experimental plot and the nearest standard weather station. However, at the same height the air temperature above wheat and barley is higher than that in the screen. In daytime, for wheat as well as for barley, the maximum difference between the calculated moisture content is 0.5 %, using the air temperature at 1.5 m height from the nearest standard weather station and the surface temperature of the ears. Barley had as a relatively high equilibrium moisture content during the night and a low one in daytime, and therefore a greater daily cycle in the moisture content of the grains than wheat.<p>The model for the calculation of the moisture content of cereals uses some crop parameters and standard (forecast) weather elements. Crop parameters used are: kind of cereals, yield and initial moisture content. Standard weather elements used are: air temperature, dew point temperature, global radiation, cloud cover, wind speed and quantity of rain. When drying, cereals lose free and absorbed water by evaporation and transpiration, respectively. Condensation and precipitation provide the grains with free water. Diffusion of free water from the reservoir in the grain provides absorbed water within the grains. The formula of Penman-Monteith is used as bases for the calculation of the moisture content. The minimum quantity of absorbed water of the grains depends on the temperature and dew point temperature of the surrounding air. This moisture content is called the equilibrium moisture content, and it is the amount of water in the grains which is bound too strongly to be evaporated. The maximum moisture content of the grains is 34% wet base.<p>Favourable agreement was found between the model calculation and the measured moisture content of cereals in the field. Usually forecast weather elements do not have exactly the same value as the corresponding past weather elements. A high moisture content of the grains can be due to a low air temperature, a low global radiation, a high dew point temperature, a high wind speed and precipitation. The quality of the moisture content forecasts of wheat and barley is decreasing with increasing time lag. Up to four days ahead the moisture content forecast is significantly useful.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Wartena, L., Promotor, External person
  • Jacobs, A.F.G., Promotor, External person
Award date22 Apr 1994
Place of PublicationS.l.
Publisher
Print ISBNs9789054852353
Publication statusPublished - 1994

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Keywords

  • drying
  • agricultural meteorology
  • models
  • research
  • fodder grasses

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