Read the following passage carefully :
Unstable weather causes considerable variations in the yields of agricultural crops. With the help of dynamic mathematical models, it is possible to forecast yields more accurately.
A dynamic model of a crop is an algorithm which permits scientists, aided by an electronic computer, to calculate within minutes a sufficiently full picture of the growth of a crop from the shoots of plants to the harvest.
The models can help to estimate the mass and areas of leaves, stalks and roots; the beginning dates of the various phases of plant growth and the amount of consumption of water and mineral substance by the crop. The scientists have succeeded in describing the influence of the environmental conditions on the physiological processes of plants such as photosynthesis, respiration, mineral feeding, growth and development. The daily changes of weather condition-air, temperature and humidity, solar radiation and rainfall-are taken into account in the calculations. A characteristic feature of dynamic models is the numerous feedbacks which make their behaviour somewhat like that of living organisms.
For instance, a sharp increase in the temperature of the air will lead to an increase in the temperature of leaves, which in turn will raise transpiration and inevitably influence the intensity of photosynthesis, the main source of building materials for the growth of plants. Since photosynthesis and transpiration will change, the crop will also change and in the future it will react differently to new temperature increases.
The picture is sufficiently complex, even when only one section of the system of relationships between weather and yields is examined. The dynamic models take a great number of such variables into account.
The method of evaluations is sufficiently simple; the scientists calculate with the help of a model the two variants of a crop-one based on factual meteorological data in a given period of time and the other founded on average climatic conditions in the same period.
Of course, dynamic models cannot fully solve the task of yield forecasting, since for this purpose scientists must have long term weather forecasts of high reliability, and this is one of the most difficult problems before modern science. Nevertheless, they can help to substantially improve agricultural practices; changes in temperatures, humidity and wind velocities can be estimated in terms of gained or lost agricultural produce.
In agronomical practice a lot of time the funds are expended on field experiments designed to test varieties and to select optimal sowing dates and the rates of seeding for given climatic conditions. Dynamic models will accelerate the solution of these tasks.
Besides, with the help of these models, scientists will be able to evaluate the influence of the possible natural and anthropogenic changes of the climate on the agricultural production.
On the basis of your reading of the above passage, answer the following questions briefly :
(a) What causes considerable variation in the yield of agricultural crops ?
(b) What is the characteristic feature of dynamic models ?
(c) How is it possible to forecast yields more accurately?
(d) What is happening in the advanced countries with regard to the yield forecast ?
