High Temperature Index Research Articles

Contribution of soil moisture variations to high temperatures over different climatic regimes

The coupling of soil moisture to climatic variables highly depends on soil moisture variability. Therefore, the activities affecting the soil moisture availability and dynamics such as land use/cover change and agricultural practices influence land-atmosphere interactions. Using soil moisture proxies, modeling studies have proven significant coupling of soil water variations to near-surface atmospheric temperature in some regions. However, these works have mostly applied the precipitation-based indices at a specific time scale. In the present study, the concurrent and lagged associations of the soil wetness variations and the high temperature index (HTI) across different climates in Iran were evaluated using Spearman’s rank test. Standardized precipitation index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) calculated at one-month and six-month scales were employed as soil moisture surrogates. When seasonal aridity index (AI) range was from 0.2 to 1.0 (transitional condition), SPEI1 and SPI1 were correlated significantly (p < 0.05) with HTI at 83.0 and 42.4 % of cases, respectively. However, when AI was below 0.2 (water-limited condition) or over 2.0 (energy-limited condition), the topsoil moisture indices showed an insignificant association with HTI at the majority of cases. A significant negative association existed between SPEI6 and HTI at most dry cases in which soil surface is dry but subsoil appears not to be fully depleted. Therefore, the topsoil and subsoil moisture variations are likely to affect high temperature occurrence under transitional and dry conditions, respectively. The insignificant lagged correlations found for most areas indicate poor linear relationship between high temperature occurrence and soil water dynamics in the preceding month over the study area. Furthermore, when AI exceed 0.2, ENSO-related soil drought is likely to cause a stronger soil moisture-temperature coupling. Overall, the role of evapotranspiration and soil moisture depth should be considered for assessing soil moisture dynamics and land-atmosphere interactions. Our findings may be helpful for adopting soil moisture-based adaptive measures to negate the impacts of high temperatures on agricultural products.

High temperature index—for field evaluation of heat tolerance in wheat varieties

High temperature (> 30 °C) at the time of grain filling is one of the major constraints in increasing productivity of wheat in tropical countries like India. Hence, wheat genotypes from national and international sources are regularly evaluated for tolerance to high temperature stress. Generally, genotypes are tested across space and time under field conditions by manipulation of date of sowing or choosing sites, which are featured by high temperature at grain filling (“hot spots”). Under such conditions, magnitude of heat stress determines accuracy of evaluation and selection of genotypes. Mean ambient temperatures during the day, month or crop season are often used to quantify the magnitude of heat stress. These temperature parameters, however, fail to account for changes in diurnal amplitude. As a consequence, actual duration of high temperature stress to which the crop is exposed to remains elusive. An attempt is made here to derive a thermal index to quantify heat stress under field conditions, taking into consideration the diurnal thermal amplitude. A mathematical equation is derived to quantify high temperature stress by integrating both the temperature and the duration. High temperature index (HTI) developed on the basis of this equation is compared with other thermal indices such as heat degree-days and mean temperatures. The index was found to be highly efficient in differentiating two crop environments. Further, HTI was used to quantify the magnitude of heat stress to which 12 advanced wheat ( Triticum aestivum) genotypes were exposed during two crop seasons. This index along with grain yield loss during hot season is used to identify genotypes performing better by tolerating the high temperature or by escaping from it.