Weather effects on crop growth, development and yield have to be quantified to interpret agronomic experiments properly and to encourage use of weather and climate information in agricultural planning and the assessment of crop yield potentials. The main objective of this research was to develop a single environmental index for use in identifying the effects of the three most important weather variables on crop production: light, temperature, and moisture. An Energy‐Crop Growth (ECG) variable was generated and tested to predict the increase in dry matter of total above‐ground corn (ZeA mays L.) plants from the time the growing point rose above the soil surface to silking. The Crop Growth Rate (CGR) was measured by destructively sampling corn plants in the field every few days in a randomized, complete‐block design, where treatments represent successive harvests in time (weather). Early and late plantings of a full‐season corn hybrid at 62,000 plants/ha on a Typic Argiaquoll at West Lafayette, Ind. from 1972 to 1974 were used to develop the relations. The ECG variable is defined as the daily product of the solar radiation intercepted by the crop canopy, a moisture stress factor and a temperature function (FT). For each unit of ECG, CGR increased about 547 kg/ha from the time the growing point rose above the soil surface to about 10 days before silking, after which it decreased to about 355 kg/ha per ECG until silking. These CGR on ECG relations were tested with independent experimental data for 1970 and 1971 to find a 1:1 relation between the predicted and measured plant dry weights. The accumulation of the temperature function alone (∑FT) was used to define the corn phenology periods examined in the development of the CGR‐ECG relations. Maximum and minimum temperatures at the 10‐cm depth in bare soil were used to calculate FT from planting to ∑FT = 12, and then air temperatures to silking. The ∑FT from planting to silking for full‐season corn hybrids averaged 37.4 ± 1.2 for 12 planting‐years, 1969 to 1974. The ∑FT method was tested for its precision and accuracy, relative to that of the modified growing degree day method (∑MGDD), by predicting dates that 50% of the corn acreage had silked in the West Central Crop Reporting District in Indiana, 1962 to 1978. The absolute error between actual and predicted silk dates averaged 3.1 ± 2.2 days using ∑FT, slightly better than the 4.1 ± 2.4 days predicted with ∑MGDD. The ECG, timed with ∑FT offers definite Potential for defining weather effects on corn growth with a single variable.

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Soil moisture balance programs developed on well‐drained soils were found to be unsatisfactory for a soil underlain by shallow water tables, a condition typical of about 9 million acres of cropland in Indiana. Capillary rise past a 105‐cm root zone boundary was estimated as the difference between estimated evapotranspiration (ET) and changes in soil moisture under corn (Zea mays L.) on a tile‐drained Typic Argiaquoll at West Lafayette, Ind. during three growing seasons, 1971–1973. Capillary water was found to supply an average of 27% of the ET in periods with little or no precipitation. Computer model estimates showed capillary water to furnish about 17% of the total ET over a 100‐day period from 49 days before silking to 50 days after.Evapotranspiration was based on measured pan evaporation adjusted with crop development and moisture stress factors from the literature. Soil moisture in the root zone was measured by neutron counting and expressed as deficits from a variable holding capacity which was allowed to change in time depending upon the depth of the shallow water table. Water table levels were measured in open wells, and water table changes were statistically related to the estimated amounts of capillary rise for use in the model. The factors used to estimate capillary rise were the soil moisture deficit in the root zone and depth of the water table.The derived relationships with those obtained from literature sources and assumptions regarding runoff and recharge were programmed in a computer model for simulating the daily moisture status and changes in the corn root zone. Model inputs were pan evaporation, precipitation, soil moisture characteristics, corn silking date, and initial soil moisture conditions. The model was found to closely track measurements of both soil moisture and water table depths in four independent seasons: early and late plantings in 1970 and 1974.

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