Global assessment of urban and peri-urban agriculture: irrigated and rainfed croplands

In contrast to previous work utilizing case studies or survey data, this study uses a spatial overlay analysis of global scale datasets to define urban and peri-urban agriculture based on the spatial coincidence of urban extents (with populations exceeding 50 000) and areas of crop cultivation. Urban aquaculture, livestock, and poultry production are relevant components of urban agriculture, but are beyond the scope of this study. Given changing boundaries of urban extent, our results likely include a mixture of urban and near peri-urban croplands. Limitations in data resolution generally exclude backyard gardens and other small-scale urban agriculture from this analysis. For the sake of brevity and consistency with terminology used in input data sources, the term 'urban croplands' is used throughout.

Recently, high-quality land use and socioeconomic datasets have been produced at the global scale. Table 1 summarizes the data sources used in this analysis [12–18] and previous studies evaluating their quality [19–24]. Further discussion of data selection metrics is included in the SI.

At the most basic level, we mapped irrigated and rainfed croplands occurring within an urban extent of more than 50 000 people through successive map overlays to identify intersections between urban extents and croplands. All analyses were conducted using ArcGIS and Python. Further details are included in the SI. Four major products for both irrigated and rainfed croplands were derived, including: (1) maximum monthly urban cropped area (all crops); (2) maximum monthly cropped area within 10 km of urban extents (all crops); (3) maximum monthly cropped area within 20 km of urban extents (all crops); and (4) annual urban harvested area (26 crop classes).

2.3.1. Identifying urban extents with populations exceeding 50 000 people

2.3.2. Identifying urban irrigated and rainfed croplands

2.3.3. Identifying peri-urban irrigated and rainfed croplands

2.3.4. Additional metrics

2.3.5. Validation

Urban croplands constitute a small, but not negligible portion of the sum of the maximum monthly irrigated and rainfed cropland area at 67.4 Mha (5.9%). However, it is important to note that this sum does not necessarily represent the total maximum monthly cropped area in instances when the maximum monthly extent of irrigated and rainfed croplands occurs in different months (i.e., irrigated and rainfed cropland areas may not be mutually exclusive month-to-month). A greater proportion of urban croplands are irrigated (35.0%) than their non-urban counterparts (17.7% irrigated) (table 2). Urban croplands also proved to be extremely prevalent globally, with 87 percent of all urban extents with populations greater than 50 000 people containing at least some area of irrigated urban cropland and 98 percent containing at least some area of rainfed urban cropland (figures S1(a) and S1(b)). While the practice of urban crop cultivation appears to be highly prevalent, we also found substantial spatial heterogeneity in the characteristics of these croplands.

The distribution of urban and non-urban irrigated and rainfed croplands varies greatly across regions (figure 1). South and East Asia comprise 49 percent of urban irrigated croplands and 56 percent of the non-urban irrigated area globally. These same two regions account for 26 percent of urban rainfed croplands and 22 percent of non-urban rainfed croplands. Developed countries account for 20 percent of irrigated urban croplands, but 44 percent of urban rainfed croplands. Sub-Saharan Africa comprises less than one percent of urban irrigated and three percent of urban rainfed croplands, but fourteen percent of non-urban rainfed croplands. Such patterns are in contrast to those of much of Asia where more croplands are located within urban extents.

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Regions with the most irrigated or rainfed cropland also dominate figures on urban croplands (figure 1). However, the actual extent of urban area in each region varies widely, making direct comparison between regions difficult. Normalizing urban cropland area by urban extent land area allows for the identification of countries where croplands constitute a larger proportion of urban land use.

The thematic maps of urban land allocation for irrigated and rainfed croplands (figures 2(a) and (b)) present distinct patterns. Regions with larger areas of irrigated cropland tend to have a higher proportion of urban extent area used for irrigated croplands. Whereas, the proportion of urban extent area used for rainfed croplands more closely parallels regional climate patterns, with more arid countries such as Namibia and Saudi Arabia having little or no rainfed urban croplands. Countries with wetter or monsoonal climates such as Rwanda and Cambodia have a greater proportion of urban land area allocated to rainfed croplands. Per capita urban cropland area by region is included in the SI (figure S2).

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The maximum monthly irrigated urban cropland area is 23.6 Mha while the sum of the annual harvested area of all crop classes is 35.0 Mha, which equates to a cropping intensity of 1.48 (table 2 and S2). In contrast, for rainfed urban croplands, the maximum monthly cropland area is 43.8 Mha (table 2 and S2) while the sum of the area of all crop classes is 45.1 Mha, suggesting fewer fields with multiple rotations.

Within urban extents, rice, wheat, and maize constitute the three major crops grown in irrigated croplands and account for 62 percent of the annual harvested area of irrigated urban croplands. In rainfed urban croplands, wheat, maize, and fodder grasses constitute the top three rainfed crops and account for 40 percent of the annual harvested area of rainfed urban croplands.

At the regional level, a more heterogeneous picture emerges. Rice, wheat, and maize still dominate the total urban irrigated cropland area, but these areas consist largely of croplands occurring in a few specific regions with extensive irrigated agriculture. Similar trends are present in the top three urban rainfed crops of wheat, fodder grass, and maize. The frequency analysis presented quantifies the number of times each crop class occurs in the top three crops produced by each of eleven regions (table 3). This approach allows for identification of the crop classes that are consistently playing a substantive role in urban crop production across regions. A complete table of the top three urban irrigated and rainfed crop classes for each region is included in table S3 in the SI. The class 'other annuals' was identified as a 'Top 3' urban irrigated crop class in 73 percent of regions. For rainfed urban croplands, 'other perennial' and 'wheat' were the two most common 'Top 3' crop classes. In general, rainfed urban croplands tended to exhibit greater diversity of crop classes and include more crops of regional significance compared to irrigated urban croplands.

The total area of irrigated croplands within ten and 20 km buffers of urban extents is 87 and 130 Mha, respectively (or 40 and 60 percent of the total irrigated cropland area of 214.5 Mha). On average, the area of irrigated cropland within ten kilometers was 3.4 times the area of urban irrigated croplands, but ranged from 2.3 times in North Africa to 5.0 times in South Asia (figure 3(a)). The area of irrigated croplands within 20 kilometers of urban extents was, on average, 5.0 times the area of urban croplands, but ranged from 2.8 times in North Africa to 8.4 times in South Asia.

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The total area of rainfed croplands within 10 and 20 km buffers of urban extents is 185 and 327 Mha, respectively (or 20 and 35 percent of the total rainfed cropland area of 931.9 Mha). On average, the area of rainfed croplands within ten kilometers was 4.3 times greater than the area of urban rainfed croplands, but ranged from 2.2 times greater in Oceania to 6.2 times greater in Sub-Saharan Africa (figure 3(b)). The area of rainfed croplands within 20 km of urban extents was, on average, 7.8 times greater than the area of urban rainfed croplands, but ranged from 3.2 times in Oceania to 13.2 times in Sub-Saharan Africa.

This analysis found an overall accuracy of 0.79 with a Kappa coefficient of 0.57 (table 4).

These findings provide the first estimate of the global extent of urban and peri-urban irrigated and rainfed croplands developed using a globally consistent, spatially explicit methodology. Urban croplands constitute up to 5.9 percent of all croplands globally. Thirty-five percent of these croplands are irrigated compared to 17.7 percent of non-urban croplands. Similarly, in peri-urban areas 60 percent of irrigated croplands were located within a 20 km buffer of urban extents, but only thirty-five percent of rainfed croplands were located within this same 20 km buffer.

This analysis also found a higher cropping intensity for both irrigated and rainfed urban croplands as compared to the total areas of irrigated and rainfed croplands. For irrigated urban croplands, the cropping intensity of 1.48 is higher than the cropping intensity of 1.12 found by Portmann et al [12] across all irrigated croplands, suggesting that farmers of irrigated croplands within urban extents are producing more rotations per year as compared to irrigated croplands overall. For rainfed croplands, the urban cropping intensity is also higher at 1.03 compared to 0.84 found by Portmann et al [12] across all rainfed croplands. The resolution of these data do not allow for disambiguation of areas producing one crop as rainfed and a second crop with irrigation. In such cases, the overall cropping intensity could be higher still.

When examined together, these findings allude to the potential significance of the topic of urban and peri-urban agriculture at the intersection of urban water management and land use planning. The new data on urban croplands could also allow further testing globally, at a fine spatial resolution, of the much discussed relationship between population density and the intensity with which croplands are farmed [27, 28]. Further work combining the urban croplands data developed through this analysis with previous work on anthromes may provide further granularity and insights into the diversity of urban croplands around the globe [29].

When food production requires irrigation, this can increase strain on water resources and create competition between domestic and agricultural water users [30]. Additional work is needed to identify the magnitude and locations where urban and agricultural water users jointly face the greatest scarcity, but, nonetheless, these findings lend preliminary insight into patterns of urban irrigated croplands across the globe and demonstrate the greater prevalence of irrigated croplands in or near urban areas.

Many of the regional trends observed with irrigated urban croplands do not hold true with rainfed urban croplands. Rainfed urban croplands are more common in developed and Commonwealth of Independent States (CIS) countries, many of which tend to have more temperate climates than the regions where irrigated urban croplands are more prevalent. Likewise, patterns in rainfed croplands generally tend to parallel climate zones more closely than irrigated croplands [31]. Many questions remain as to how the extent and contribution of urban rainfed croplands will adapt to the dual stressors of urban growth and climate change.

Regional trends in crop production (particularly rainfed urban croplands) appear to be closely connected to regional markets, but disambiguation of specific drivers is beyond the scope of this analysis. More generally, while staple crops constitute a large portion of urban croplands, other annuals are a dominant urban crop in 73 percent of regions; a finding consistent with past reviews of urban agriculture [2, 3, 32]. Whether these crops are destined for local, regional, or international markets and, in turn, their contribution to local food security is likely a function of access to supply chains, transport, and processing facilities.

Understanding the role of urban and peri-urban crop production in urban food security at scale remains a major knowledge gap in the field of urban agriculture. Using a more conservative estimate of urban extent boundaries, Martellozzo et al [11] estimate that a mean of thirty percent of the global urban extent area would need to be allocated to meet actual urban vegetable consumption. However, this value varied substantially between countries ranging from 1.2 to 397.4 percent or urban extent land area. In contrast, this study found countries allocating an average of 4.8 percent (SD 8.3) and 13.4 percent (SD 13.3) of urban extent areas towards urban irrigated and rainfed croplands, respectively. In both studies the role of small and medium urban extents was substantial. While these two studies are not directly comparable due to differences in the urban extent boundaries used and the differing metrics of urban vegetable demand and urban cropland extent, these findings nonetheless suggest that urban and peri-urban croplands are playing a non-negligible role in meeting urban food demands at the global scale.

While this analysis provides the first global estimate of the extent and distribution of urban and peri-urban croplands developed using globally consistent methods, the issue of spatial resolution remains a central consideration when interpreting these results. Specifically, the scale and methods used to develop the input cropland data are not structured to capture very small, spatially dispersed areas of urban croplands. Therefore, the types of urban cropland captured through these methods are, generally speaking, croplands over slightly larger areas occurring along the urban periphery. Given these exclusions, we would estimate that the areas of urban croplands found in this analysis are fairly conservative and underestimate the actual area of urban and peri-urban crop production. Additional case studies of urban agriculture focusing on urban and peri-urban plots found higher proportions of vegetable production [33–35] than were found in the types of urban croplands captured in our analysis suggesting that this analysis may also be underestimating the extent of vegetable and fruit production in urban croplands.

Urban boundaries are also frequently not static nor explicit. Taylor et al [24] found that the GRUMP data provide a generous estimate of urban extent boundaries when compared to MODIS 500 and other urban land cover data. This finding was further confirmed while conducting the validation analysis in this study. Since the focus of this analysis is identifying croplands within or along the urban periphery, this difference simply means this analysis is capturing one of many possible points along the urban to peri-urban continuum. The implication for this research is that the estimated urban cropland areas presented in this paper include a mixture of urban and peri-urban croplands. Further details on data analysis and limitations can be found in the supplemental information, available at stacks.iop.org/ERL/9/114002/mmedia.