The mean annual precipitation is below 250 mm in about 70% of the country and only 3% of Iran, i.e. 4.7 million ha, receives above 500 mm yr−1 precipitation . The geographical distribution of Iran’s croplands shows that the majority of Iran’s cropping activities take place in the west, northwest, and northern parts of the country where annual precipitation exceeds 250 mm . However, irrigated cropping is practiced in regions with precipitations as low as 200 mm year−1 , or even below 100 mm year−1 . To support agriculture, irrigated farming has been implemented unbridled, which has devastated the water scarcity problem. challenges: providing domestic food to a rapidly growing population on a thirsty land.When land suitability was evaluated solely based on the soil and topographic constraints , 120 million ha of land was found to have a poor or lower suitability ranks . Lands with a medium suitability cover 17.2 million ha whilst high-quality lands do not exceed 5.8 million ha . The spatial distribution of suitability classes shows that the vast majority of lands in the center, east and, southeast of Iran have a low potential for agriculture irrespective of water availability and other climate variables . As shown in Fig. 2, the potential agricultural productivity in these regions is mainly constrained by the low amount of organic carbon and high levels of sodium contents . Based on soil data, Iran’s soil is poor in organic matters with 67% of the land area estimated to have less than 1% OC. Saline soils, defined by FAO as soils with electrical conductivity >4 dS/m and pH<8.2, are common in 41 million ha of Iran. Although many plants are adversely affected by the saline soils , there are tolerant crops such as barley and sugar beet that can grow almost satisfactorily in soils with ECs as high as 20 dS/m,nft growing system which was used as the upper limit of EC in this analysis .
Although sodic soils are less abundant in Iran , soils that only have high ESP covers ~30 million ha . We used an ESP of 45% as the upper limit for cropping since tolerant crops such as sugar beet and olive can produce acceptable yield at such high ESP levels. As shown in Fig. 2, EC is not listed among the limiting factors, while we know soil salinity is a major issue for agriculture in Iran. This discrepancy can be explained by the higher prevalence of soils with ESP>45% compared to those with EC>20 dS/m, which can spatially mask saline soils. That is, the total area of soils with EC>20 dS/m was estimated to be about 6.4 million ha , while soils exceeding the ESP threshold of 45 constituted ~12 million ha i.e. almost double the size of saline soils. Iran’s high-quality lands for cropping are confined to a narrow strip along the Caspian Sea and few other provinces in the west and northwest . In the latter provinces, the main agricultural limitations are caused by high altitude and steep slopes as these regions intersect with the two major mountain ranges in the north and west .Thus far, the land suitability analysis was based on soil and terrain conditions only, reflecting the potential agricultural productivity of Iran’s without including additional limitations imposed by the water availability and climatic variables. However, Iran is located in one of the driest areas of the world where water scarcity is recognized as the main constraint for agricultural production. Based on aridity index , our analysis showed that 98% of Iran could be classified as hyper-arid, arid, or semi-arid . August and January are the driest and wettest months in Iran, respectively, as shown in Fig. 3. Over half of the country experiences hyper-arid climate conditions for five successive months starting from June . This temporal pattern of aridity index has dire consequences for summer grown crops as the amount of available water and/or the rate of water uptake by the crop may not meet the atmospheric evaporative demand during these months, giving rise to very low yields or total crop failure. It must be noted that the high ratio of precipitation to potential evapotranspiration in humid regions could also result from low temperature rather than high precipitation.
There is a high degree of overlap between regions that experience wetter conditions for most of the year and those identified as suitable for agriculture based on their soil and terrain conditions . This spatial overlap suggests that some of the land features relevant to cropping might be correlated with the climate parameters. In fact, soil organic carbon has been found to be positively correlated with precipitation in several studies. To incorporate climate variables into our land suitability analysis, we used monthly precipitation and PET as measures of both overall availability and temporal variability of water. We derived, from monthly precipitation and PET data, the length of the growing period across Iran . Growing period was defined as the number of consecutive months wherein precipitation exceeds half the PET. As shown in Fig. 3, areas where moisture conditions allow a prolonged growing period are predominately situated in the northern, northwestern, and western Iran with Gilan province exhibiting the longest growing period of 9 months. For over 50% of the lands in Iran, the length of the moist growing period is too short to support any cropping unless additional water is provided through irrigation . However, these areas, located in the central, eastern, and southeastern Iran, suffer from the shortage of surface and groundwater resources to support irrigated farming in a sustainable manner. Taking into account daily climate data and all types of locally available water resources can improve the accuracy of the length of growing period estimation. The productivity of rainfed farming is also affected by the selection of planting date, which often depends on the timing of the first effective rainfall events. For this joint soil-terrain-climate analysis, all regions with a growing season of two months or shorter were assigned a suitability value of zero and thus classified as unsuitable for agriculture. We then evaluated the capacity of land for rainfed farming by using a precipitation cut-of of 250 mm year−1 ,vertical hydroponic nft system which is often regarded as the minimum threshold for the rainfed farming . As shown in Table 1, the inclusion of the length of growing period and precipitation threshold into the analysis only slightly reduced the total area of high-quality lands from 5.8 to 5.4 million ha. This implies that most lands with suitable soil and terrain conditions also receive sufficient amount of moisture to sustain rainfed agriculture.
On the contrary, the area of unsuitable lands increased from 39.7 to 112.9 million ha when precipitation and duration of growing season thresholds were superimposed on the soil and topographic constraints. This increase in unsuitable acreage was mainly driven by the demotion of lands from the very poor class to the unsuitable class . The addition of moisture constraints also reduced the area of medium suitability lands by 4.8 million ha. In summary, for the rainfed farming suitability analysis, 125 million ha of Iran’s land might be classifed as poor or lower ranks whilst only 18 million ha meet the required conditions for the medium or higher suitability classes . Te geographical distribution of these land classes is mapped in Fig. 4. Almost the entire central Iran , and the vast majority of land area in the eastern , southeastern and southern provinces were found to be unsuitable for rainfed farming. Almost half the area of Khuzestan and three-quarters of Fars provinces were also characterized unsuitable. Over the entire east, only in the northern part of Khorasan Razavi province, is there a belt of marginally suitable lands satisfying the requirements of a potentially prosperous rainfed agriculture .In the next step of the analysis, the suitability of land was scaled with the annual precipitation over the range of 100 to 500 mm year−1 . The lower limit is deemed to exclude the desert areas for agricultural use whilst the upper limit represents a benign moisture environment for the growth of many crops . This last analysis, here after referred to as precipitation scaling method, makes no assumption as to whether the cropping practices rely on rainfall or irrigation to satisfy crop water requirement and may thus represent a more comprehensive approach for agricultural suitability assessment. The same minimum length of growing period and soil/topographic constraints as with the two previous methods were used in this analysis. Compared to the rainfed agriculture analysis, the precipitation scaling method mainly changed the distribution of lands within the lower suitability classes . For example, a great proportion of lands within the unsuitable class was shifted up to the very poor and poor classes. This implies that, to a limited extent, irrigation can compensate for the below threshold precipitation . Nevertheless, water availability cannot necessarily justify agriculture in areas with low soil and topographic suitability. This has an important implication for water management in Iran that has a proven record of strong desire for making water available to drier areas through groundwater pumping, water transfer, and dam construction. The majority of high-quality lands , which also retains sufficient levels of moisture are located in the western and northern provinces of Iran . Kermanshah province accommodates the largest area of such lands followed by Kurdistan .
High-quality lands were estimated to cover 33% and 21% of these two provinces, respectively. Other provinces with high percentages of high quality lands were Gilan , Mazandaran , West Azerbaijan , and Lorestan . For 17 provinces, however, high-quality lands covered less than 1% of their total area .To estimate the total area of croplands within each suitability class, we visually inspected 1.2 million ha of Iran’s land by randomly sampling images from Google Earth . The proportion of land used for cropping increased almost linearly with the suitability values obtained from the precipitation scaling method . Total cropping area in Iran was estimated to be about 24.6 million ha, which is greater than the reported value by the Iran’s Ministry of Agriculture. This authority reports the harvested area; hence, the fallow or abandoned lands are not included in their calculation of active agricultural area. Our visual method, however, captures all lands that are currently under cultivation or had been used for cropping in the near past that are now in fallow or set-aside . The relative distribution of croplands amongst the suitability classes shows that about 52% of the croplands in Iran are located in areas with poor suitability or lower ranks as identified by the precipitation scaling method. Particularly concerning are the 4.2 million ha of lands that fall within the unsuitable class. Approximately 3.4 million ha of cropping areas occur in good and very good lands . However, no agricultural expansion can be practiced in these areas as all available lands in these suitability classes have already been fully exploited. Medium quality lands comprise 12.8 million ha of Iran’s land surface area , of which about 8.6 million ha have been already allocated to agriculture . Nevertheless, due to their sparse spatial distribution and lack of proper access, only a small portion of the unused lands with medium suitability can be practically deployed for agriculture. Using FAO’s spatial data on rainfed wheat yield in Iran, we estimated the mean yield for wheat cropping areas located within each of the six suitability classes. As shown in Fig. 7, the yield of the rainfed wheat increased proportionally with improving suitability index, showing that our suitability index adequately translates to crop yield. Using the observed yield-suitability relationship , we estimated that 0.8 million ton of wheat grain might be produced per year by allocating 1 million ha of the unused lands from the medium suitability class to rainfed wheat cropping.Whilst the insufficiency of water resources has long been realized as a major impediment to developing a productive agriculture in Iran, our study highlights the additional limitations caused by the paucity of suitable land resources.That is, Iran as a member of Convention on Biological Diversity is obliged to fulfil Aichi Biodiversity Targets whose Target 11 requires Iran to expand its protected area to 17% by 2020, which is almost double the size of the current protected areas in Iran .