نشریه هواشناسی کشاورزی
سال پنجم شماره 2 (پاییز و زمستان 1396)

Over the last few decades, the water level appears to have declined for most of the Lakes as a consequence of global warming. Besides, in addition to climatic factors, human activities have had profound impact on lowering lakes water level and ecosystem changes. The aim of this study was trend analysis of climatic factors (rainfall and evapotranspiration) and water balance (flow and underground water withdrawal) in Maharloo Lake region during last five decades using Man Kendall and Petit Tests. The results showed a %82 decrease of Lake water level. Rainfall did not show a significant trend at %1 significance level , but there exist a significant trend for evapotranspiration which might be a major cause for observed reduction of water level. Human activities and increased consumption and withdrawal in Maharloo basin may be considered as the other major affecting factors. Such that the discharge of Khoshkrood river which flows into the lake has significantly decreased since 2006.

As a result of global warming a significant change in wet and dry spells pattern is expected. These variations would be more significant in regions with diverse climates like Golestan province, Iran. In this research, the impact of climate change on the frequency and intensity of droughts are assessed using Standardized Precipitation Indices (SPI) in two raingauges stations namely Arazkuse and Tamar. The rainfall data for baseline period (1986-2005) for both stations were collected and examined. The projections of rainfall amount for two future periods, 2031–2050 and 2051–2070, obtained from the Coupled Model Intercomparison Project Phase 5 (CMIP5) outputs were downscaled under three representative concentration pathway (RCP2.6, RCP4.5 and RCP8.5), using the statistical downscaling model (SDSM). After evaluation of skill of WG model in simulation of historical rainfall data, SPI and relative SPI values in different time steps were calculated. Then by using first-order Markov chains drought characteristics during baseline and future period were compared. The results showed that probability of droughts in the future in both Tamar and Arazkuse stations, would increase. With increasing SPI time scale, duration of all drought classes is projected to decrease in the future under all three RCP scenarios.

Lack of regional climatic data is great challenge for agroclimatic studies. In this research, the influence topography on RegCM4 model results was investigated. In order of determine the role of topography on improving the results of RegCM model, two topographic datasets (GTOPO, GMTED) were used for a specific period of time and similar conditions to run the model. The experiments were done with lateral boundary conditions obtained from The NCEP-DOE Reanalysis 2 (NNRP2) dataset at 2.5° × 2.5° latitude-longitude horizontal resolution. The sea surface temperature (SST) for the corresponding regional climate simulations were prescribed from the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation SST (OISST) dataset with a weekly temporal resolution and 1° × 1° spatial resolution. The topographical output of the model showed an increase of elevation in the Caspian sea coast [max (GMTED-GTOPO) = 254 m], while in some areas of Alborz mountains a decrease in elevation [max (GMTED-GTOPO) = -400 m] was observed. Due to higher elevation of GTOPO in the Alborz Mountains, the derived speed of this simulation is relatively higher than GMTED, which could reach as high as 1.6 m/s during winter season. The highest precipitation difference occurred during spring season in the vicinity of the area where 400 m of decreased elevation was observed, with amounts as high as 16 mm/day (80% increase in precipitation). The temperature differences followed closely the topographical differences. In the seashore areas, where there was increased elevation, temperature decreased correspondingly (max. of -2.4°), whereas in mountainous areas with decrease in elevation, the temperature increase was observed (max. of 1.2°). The findings from this research identified, the modified data (GMTED) is improving the results of the model and should be replace the old data (GTOPO) in future research.

Drought is one of the most damaging natural hazards that it is placed in the first order in terms of both the frequency of occurrence and the magnitude of created damages. In this study, we used the standardized precipitation - evapotranspiration drought index (SPEI) for analysis of drought in the 54 synoptic stations for a 64 years (1951- 2014) period. For drought monitoring and regionalization, the SEPI values were calculated in 12, 24 and 48 months timescales. The trend of rainfall, temperature and SEPI was investigated using Mann-Kendall test during the study period. The results showed that an increasing trend for drought and temperature and decreasing trend for rainfall. Considering the correlation of drought and temperature, the findings of study confirmed that SEPI had a good performance for monitoring drought across Iran.

Considering the significant increase of pistachio cultivation area in Iran and lack of water resources, accurate determination its water requirement is necessary. Since irrigation in the initial stages of pistachio is essential for adequate tree establishment, the main objective of this study was to measure the evapotranspiration of pistachio trees (ETc) between four to nine years of age, when the economical yields is achieved. This study was conducted in Rafsanjan Pistachio Research Institute station using a cubic concrete drainage lysimeter with a 54 m2 surface area (6 m × 9 m) and a depth of 3.4 m. One-year old seedlings of Pistacia vera L. were cultivated inside and surrounding of the lysimeter. The amount of evapotranspiration and crop coefficient (Kc) was measured every 20-days during the eight-month growing season of pistachio using lysimeteric data. For the young trees For the young trees from four to nine years of age, the amount of seasonal evapotranspiration was 2291.2, 2507.8, 2739.4, 4516.0, 4604.0 and 5621.0 cubic meters per hectare with the mean Kc values of 0.108, 0.104, 0.15, 0.21, 0.28 and 0.33, respectively.The mean Kc coefficients of pistachio in five growth stages were determined as 0.2, 0.17, 0.22, 0.16 and 0.19, respectively.

In this study the relationship between annual Standard Precipitation Index (SPI) and, remotely sensed indices of Enhanced Vegetation Index (EVI) and Normalized Differencing Vegetation Index (NDVI) have been studied. MODIS images of Terra satellites for the period of 2000 to 2012, during the growing season of pastures across Golestan province alongwith rainfall data of five meteorological stations in Golestan rangelands from their establishment till 1993 were collected. Pearson correlation coefficient was used to assess the relationship between rainfall data and vegetations indices. Correlation coefficient between (SPI) drought index and EVI and NDVI indices was 0.75 and 0.78 respectively at 5% level of significance. Bedsides, correlation between the NDVI and EVI indices in detection of the vegetation changes were 0.98. In general, the results of this study showed that in regions with inadequate rainfall data, remote sensing products can be used for drought and vegetation cover dynamic monitoring.