Study area

The study was developed for the growing conditions of the agricultural region of Hermosillo, Sonora, Mexico. The city of Hermosillo is located west of the State of Sonora, Hermosillo is the capital located between parallels 29° 10' and 28° 59' north latitude and between the meridians 111° 4' and 110° 55' west longitude at an altitude of 282 masl; bordered on the northeast by Carbo and San Miguel de Horcasitas; to the east Ures and Mazatlan; southeast La Colorada and Guaymas; northwest Pitiquito and southwest with the Gulf of California (Figure 1).

Figure 1 Location of the study area in the city of Hermosillo, Sonora. 

Calculation method for cold hours

Two methods accuracy regarding the estimation of cold hours in winter were evaluated:

a) Da Mota method (^{Diaz, 1987}), which calculates the monthly accumulated cold hours as follows:

HF= 485.1 - 28.52 Xi

Where: Xi is the average temperature for the months of November, December, January and February.

b) Weinberger method (^{Ortiz, 1987}), which relates the number of cold hours accumulated with the average of mean temperatures from December and January, with the following equation:

HF= 2124.85 - 125.23 Xi

Where: Xi is the average of the mean temperatures from December and January (°C).

The assessment of the accuracy of these methods was made comparing the values of accumulated cold hours with this methodology against total cold hours total accounted directly through ten-minute data from automated agro-meteorological station CECH-INIFAP from the Agroson Network (www.agroson.org.mx). Calculations and assessment of the two indirect methods were made taking into account a series of ten winter cycles (2004-2005 to 2013- 2014). The direct calculation of cold hours was performed by counting effective cold hours (HFE), using the following expression, which will be called the INIFAP method:

HFE= HF-HC

Where: HF are the hours with temperatures> 0 and ≤ 10°C; while HC are the hours with temperature> 25°C.

The adjustment of indirect methods to calculate cold hours were assessed through Pearson ́s correlation coefficient, establishing the correlations Da Mota vs INIFAP and Weinberger vs INIFAP. The indirect method that best correlated with INIFAP method was selected.

Climate change scenarios

Once selected the indirect method of best accuracy, it proceeded to estimate the accumulation of cold hours for climates 1961-2010 (reference), 2030, 2050 and 2070 under the representative concentration pathways of greenhouse gases (RCP) 4.5 and 8.5. For future climate scenarios the information climate change system was used (^{Ruiz et al., 2005}), derived from the assembly of 11 general circulation models: BCC-CSM1-1, CCSM4, GISS-E2-R, HadGEM2-AO, HadGEM2-ES, IPSL-CM5A-LR, MIROC-ESM-CHEM, MIROC-ESM, MIROC5, MRI-CGCM3, NorESM1-M. The implementation of these calculations was performed with map algebra routines through Idrisi Selva system. As reference climatology the period 1961-2010 was used, for which the interpolated climate information system by the INIFAP originally at a resolution of 90 m was used. All this information was used and managed in raster images with a resolution of 30 "arc.

The images obtained from cold average temperature from December to January and accumulated cold hours in all climate scenarios considered were compared. Finally image edition to convert them into maps was made through ArcGis system, where image reclassification procedures were considered according to intervals determined in accordance with the minimum and maximum values of thematic images.

Evolution of average temperature in the period from December to January

Figure 2 shows the dynamics of the average temperature with Rcp 4.5, note that currently there is an average between 14 and 16 °C, but immediately in the scenario from 2030 the temperature leaving only small remnants in the end portion of the north (36th Street North), but as scenarios move forward, no longer records an area in Hermosillo region, in a thermal range below 16-18°C, showing a substantial increase in temperature, which will definitely affect the accumulation of winter cold for deciduous fruit trees such as pecan. The latter will affect adjacent municipalities such as San Miguel de Horcasitas and Pitiquito that could be alternative areas to grow pecan in the future.

Figure 2 Scenarios maps for current and future average temperature 2030, 2050 and 2070 in the Rcp 4.5 for the pecan region in Hermosillo, Sonora. 

Moreover, Figure 3 shows the scenarios obtained for Rcp 8.5, which shows that since scenario 2030 the temperature will increase to the range 18-20°C (+ 4 °C) and even on scenario 2050 much of the pecan region from Hermosillo, especially the eastern part of the agricultural area will have an average temperature in the range of 20-22°C, but for climatology 2070, will be seen that all the pecan region will have a mean temperature range of 20-22°C and even some areas of the west of the city will reach the range of 22-24°C (+8°C).

Figure 3 Scenarios maps for current and future average temperature 2030, 2050 and 2070 in the Rcp 8.5 for the pecan region in Hermosillo, Sonora. 

Evolution of cold hours in the period from December to January

Figure 4, shows the dynamics of cold hours for Rcp 4.5 hours. Current climatology (1961-2010) shows that the accumulation oscillates in the range of 200-300 cold hours (HF), for most of the pecan region in Hermosillo, Sonora, and elsewhere (Eastern Section) in the 1 range 100-200 HF, which fills a minimum requirement of cultivars currently used (Western and Wichita) which involves the use of sprouting promoters in pecan plants, but in the next scenario (2030) cold accumulation drastically decreases to 0-100 HF in most of the region, leaving a small remnant of 100-200 HF in the north end of the Costa de Hermosillo, which does not cover the requirements of these cultivars, because as mentioned by ^{Smith et al. (1992)}, some similar cultivars to the latter like Mahan require between 300 and 400 HF. Even regions that are currently considered as alternative areas for future establishment of pecan plantations, like San Miguel de Horcasitas and Carbo, will present a low capacity for cold accumulation in 2050 and 2070, as it is estimated an accumulation between 0 and 100 HF in the winter season. According to these results it could be consider that the use of current pecan cultivars would no longer be viable in the future.

Figure 4 Scenarios maps for current and future cold hours in 2030, 2050 and 2070 in RCP 4.5 for the pecan region of Hermosillo, Sonora. 

As for Rcp 8.5, HF accumulation on scenario 2030 onwards (Figure 5) would be in the range of 0-100 HF, which is too low for varieties of current use. This would be the worst climate scenario for the region, as it would be virtually eliminating pecan and overall deciduous fruit from the regional pattern of cultures, from 2030.

Figure 5 Scenarios maps for current and future cold hours in 2030, 2050 and 2070 in RCP 8.5 for the pecan region of Hermosillo, Sonora.