Study area

The Carabobo state is located in the northerncentral region of Venezuela between the geographic coordinates 09°48' and 10°35' of latitude N, and 67°31' and 68°26' of longitude W (figure 1); The total area is 4,651 km² of which only 42.53 % (1,973 km²) is suitable for the development of agricultural activities.

The average annual rainfall in the Carabobo state is 1,100 mm. Accumulated rainfall ranges between 1,100 and 1,300 mm in the depression area of Lake Valencia, formed by low mountains, hills, and valleys that drain through alluvial and lake plain reliefs (^{Olivares, 2018}). Most of the territory is categorized by having a dry sub-humid (C1) climate comprising an approximate area of 2,791.15 km², i.e., 60% of the region (^{Olivares, Hernández, Cohelo, et al., 2018a}).

From an agrological point of view, the highest proportion of soil in Carabobo is classified as Class I, followed by the other classes up to Class VIII soils. These classes represent the soils with ideal characteristics or that are less limiting for the agricultural production in the territory that covers an area of 103,700 ha (^{Comerma, 2009}).

Source: Elaborated by the authors

Figure 1. Geographic location of the Carabobo state in the central region of Venezuela. 

Methodological components of the ecoterritorial sectorization

The ecoterritorial sectorization is a resource of particular interest for the agricultural development of a study area because optimum environmental conditions are established for a plant species where its maximum production potential is feasible. Namely, it is possible to interrelate the factors that affect the development of the crop such as climate, soil, relief, and agroclimatic requirements, amongothers, to establish a map showing the suitabilitydegree of the land.

Figure 2 describes how to apply the procedures included in the ecoterritorial sectorization methodology, which attempts to estimate land suitability and potential productivity focusing on potato (S. tuberosum) production systems.

Source: Elaborated by the authors

Figure 2. Description of the activities and the methodological components of the ecoterritorial sectorization of potato (S. tuberosum) in the state of Carabobo, Venezuela. 

Climate analysis phase

To elaborate water balances for potato, the analysis of decadal precipitation (accumulated precipitation of 10 days) in the Carabobo state for the period between years 1970 and 1999 and corresponding to 41 meteorological stations, was carried out. Daily data series were filtered, complemented, and then converted to decadal data. These procedures were carried out through the quality control of meteorological information, as well as by using statistical analyses and methods already established for the estimation of missing and aggregate daily data (^{Hernández et al., 2017}; ^{Olivares, Cortez, Parra, Rodríguez, & Guevara, 2013}; ^{Parra & Cortez, 2005}).

Nonparametric tests to verify the homogeneity of the precipitation and evaporation data were applied. Daily historical precipitation records were obtained by calculating missing data using correlation methods, distances, and normal values; similarly, for their apportionment, the nearest neighbor method was used (^{Olivares et al., 2013}). Regarding missing daily air surface evaporation data (type A tank), regional homogeneity and the nearest neighbor methods were applied.

Reference or potential crop evapotranspiration (ETp) was calculated using the Penman-Monteith FAO method (^{Allen, Pereira, Raes, & Smith, 2006}). Subsequently, to establish the water supply and demand of suitable agricultural lands in the Carabobo state, the water balance of the crop for each climatological station was calculated using the programming language Clipper (^{Hernández et al., 2017}), which provided information to perform an edaphoclimatic risks analysis.

Clipper calculated an average of 42 water balances for potato, assuming as the sowing date each of the decades of the year. Furthermore, it estimated the effective precipitation from the slope of the terrain, and it calculated the water storage in the soil during three decades before the planting date to ensure an adequate moisture content at that time. Also, it calculated the decrease in yield by deficit using the Ky index, and the reduction in yield by excess employing the Food and Agriculture Organization method ( ¹¹); the latter was used because it is a practical and useful methodology adapted to the tropical conditions of the productive systems and ecological interactions.

The methodology published by Franquín (1983) was developed for the analysis of the start dates, duration, and water layers accumulated through rainfall for the growth and humid periods, according to the probability of occurrence levels of 50 % and 75 %. This information is useful to reduce the uncertainty of the agricultural activities, as well as in the growth stages of the crop that are related to their water requirements.

Edaphic analysis phase

The land-use aptitude classification map of the Carabobo state elaborated by Dirección General de Recursos Hidráulicos of Ministerio de Obras Públicas (1971) at a scale of 1: 100,000 was used as a base for the edaphic analysis, as it is the only soil study available (^{Hernández et al., 2017}). The edaphic qualities assessed were the type of drainage, the depth, the soil texture and the slope of the terrain, essential variables to calculate the maximum soil storage capacity (mssc).

Additionally, also data from the Information System of Agroecological Areas (siaa, for its acronym in Spanish) of Instituto Nacional de Investigación Agrícola (inia, for its acronym in Spanish) [National Institute of Agricultural Research] ( ¹⁷), and the Soil Information System of the Depression of Lake Valencia (Sisdelav, for its acronym in Spanish) (^{Viloria, Estrada, & Rey, 1998}) were used.

Evaluation phase of crop requirements

Crop requirements were defined using the contributionsof Benacchio (1982), who compiled thewater requirement, the temperature and other climaticfactors that affect the development of differentcrops in tropical environments (table 1). Cropcoefficient values (Kc) were taken from Doorenbosand Kassam (1979) and Doorenbos and Pruitt(1975).

Table 1. Edaphoclimatic and nutritional requirements for potato (S. tuberosum) cultivation 

Source: Adapted from Benacchio (1982) and INIA (2016)

The phenological phases of potato considered and described below are the ones indicated by Hack et al. (1992):

Potato plants are resistant to drought during their phenological phases, which is reflected by the Kc index. Table 2 shows the Kc index values for this crop.

Table 2. Crop coefficients (Kc) in a growth cycle of 140 days 

Source: Adapted from Doorenbos and Kassam (1979) and Doorenbos and Pruitt (1975)

Water requirements vary from 1,000-1,200 mm and are well distributed during the crop cycle. The highest water demands if found in the germination and tuber growth stages, so it is necessary to irrigate the crop in these critical periods, especially when no rainfall occurs. The final crop development stages are the most susceptible to water deficit, and in these, the yield can be reduced considerably depending on whether this deficiency occurs in the initial stages; besides, these do not tolerate drought for long periods.

Ecoterritorial sectorization

The main objective of this methodology is to compare crop requirements in terms of soil and climate, and the edaphoclimatic "supply" of the area, to quantify how satisfactorily the requirements are covered and, consequently, the degree of suitability that the area has for potato cultivation. Accordingly, first, soil, climate, and crop relationships are analyzed separately, and then they were integrated to obtain a single result (^{Ministerio del Ambiente y los Recursos Naturales Renovables [marnr], 1989}; ^{Olivares, Hernández, Arias, Molina, & Pereira, 2018a}).

The result of the water balance allowed analyzing the maximum crop evapotranspiration (ETm), the crop yield, the yield response factor to water (Ky), the crop coefficient (Kc), the relation between the water demand of the crop (real crop evapotranspiration [ETa]) and the demand of the reference crop (ETp). Moreover, also the yield percentage for an estimated sowing date that guarantees an initial soil humidity.

Furthermore, fao (2012) addressed the relationship between crop yield and water use and proposed a simple equation that related the relative decline in yield with the corresponding relative decrease in evapotranspiration (et). Specifically, the yield response to et is expressed in equation 1.

Where Ya is the real yield harvested, Ym is the maximum yield harvested, Ky is the yield response factor to water, ETa is the real crop evapotranspiration, and ETm is the maximum evapotranspiration of the plant.

The Ky factor captures the essence of the complex relationships that exist between production and water use in a crop, where biological, physical and chemical processes occur. The relationship has shown remarkable validity and has provided a procedure that is useful to quantify the effects of water deficits on yield ( ¹²; ^{Olivares, Hernández, Arias, Molina, & Pereira, 2018b}).

According to fao (2012), the procedure for calculating equation 1 to establish the real yield is comprised of four steps: i) estimating the maximum yield (Ym) of an adapted crop variety according to its genetic behavior and the climate, assuming that agronomic factors (e.g., water, fertilizers, pests, and diseases) are not limiting. ii) calculate the maximum evapotranspiration (ETm) according to established methodologies and bearing in mind that all the water requirements of the crop have been supplied. iii) calculate the real evapotranspiration (ETa) of the crop in a specific case, according to the water supply available for the plant, and iv) evaluate the real yield (Ya) by appropriate selection of the response factor (Ky) throughout the growth period or in each of the different growth stages.

The faoIrrigation and Drainage N.° 33 study (^{Doorenbos & Kassam, 1979}) recommended procedures to estimate the maximum yield from available local data on maximum crop yields or based on the calculation of maximum biomass and the corresponding harvest index, using the ecological zone approach (^{Kassam, 1977}). Subsequently, a general adaptability criterion was established according to the Ya/Ym relationship (table 3).

Table 3. Adaptability ranges according to the ratio between the real yield harvested and the maximum yield harvested (Ya/Ym) 

Source: Elaborated by the authors

Then, the water requirements of the crops (ETa) were calculated for each suitability range, both for the vegetative cycle as well as for the critical stages, and were compared with the climatic supply of the study area, defined as the effective precipitation with 75% probability of occurrence. Subsequently, a total climatic suitability index (equation 2) was calculated to establish the degree of suitability (table 4).

Where tiis the total index, vci is the vegetative cycle index, and cpi is the critical phase index.

Table 4. Suitability ranges according to the climate 

Source: Elaborated by the authors

An interpolation to map the edaphoclimatic variables was performed using the punctual Kriging technique. Further, a variation model was established using geostatistics (^{Webster & Oliver, 1990}), which allowed defining the spatial structure of the properties and their estimation in sites not sampled by the Kriging interpolation method. The estimates were calculated using the semivariogram parameters adjusted by equation 3.

Where n is the number of samples neighboringZ(x₀); λ is the relative weight applied to each Z(x₁);Ž(x₀) is the estimate of Z in x₀, and Z(x_i) is the samplenext to x₀.

Once the file with the interpolated variables was generated, it was edited in the SIG program ArcView v.3.2., to be visualized, analyzed and classified (Environmental Systems Research Institute [ ⁹). This procedure allowed examining the spatial variation of suitable or unsuitable areas for the crop under study.

Land suitability

The superficial extension of the soils that is suitable for potato cultivation in the municipality of Libertador in the state of Carabobo is 90 km²; therefore, it is considered as the municipality with the most significant extension of arable lands for potato cultivation. The average annual rainfall ranges from 1,500 mm in the south to 1,000 mm in the north that corresponds to the depression of Lake Valencia and Serranía del Litoral. Table 5 presents the results of the ecoterritorial sectorization of potato in the state of Carabobo.

The estimated sowing dates for the cultivation of potato in suitable soils in the central-northern area were found between the third decade of May and the first decade of August. To the north of the town of Tocuyito, optimal sowing dates are found between the first decade of June and the first decade of August. On the other hand, to the northwest, the suitable sowing dates correspond to the second decades of June and of July.

Towards the south of the municipality of Libertador, the sowing date in some sectors was estimated between the second decade of June and the third of August; finally, in terms of suitable soils, the sowing date in some central and southeastern areas was estimated between the first and third decades of July.

It is convenient to indicate that, from the moment the plants are sown, the moisture content of the soil exerts a considerable influence throughout their development cycle. When there is excess water, the percentage of starches decreases and the appearance of certain pests, diseases, and weeds are favored. On the other hand, variations in the moisture content generate modifications in the thickness of tubers and the presence of cracks, furrows, constrictions, among other malformations. Studies report that the occurrence of water deficit before tuberization stimulates root lengthening, increasing the number of tubers significantly but their size is reduced considerably (^{Morales, Morales, & Rodríguez, 2015b}).

Table 5. Land surface (km²) suitability for potato (S. tuberosum) cultivation per municipality in the state of Carabobo 

Source: Elaborated by the authors

The occurrence of drought in the Venezuelan agricultural territories affects the vegetative growth of potatoes and inhibits tuberization, in particular, size and quality (^{Cortez et al., 2018}, ^{Parra et al., 2018}). The critical periods of water needs in potato cultivation occur immediately after emergence and during tuberization (^{Morales et al., 2015b}; ^{Pino, Inostroza, Kalazich, Gutiérrez, & Castro, 2012}).

Regarding the moderately suitable soils for the cultivation of potato, these cover an area of 92 km² of the agricultural lands of the municipality.

The prevailing sowing dates are found between the second and third decades of May, as shown in figure 3. In some areas of the southern and central regions of the municipality of Libertador, the date was estimated between the third decade of May and the third of June. In specific areas of the northern region, the sowing dates were between the first decade of June and the third decade of July; in some sectors towards the northwest, it was estimated solely for the first decade of July, meanwhile in others, just in the second decade of July.

Source: Elaborated by the authors

Figure 3. Ecoterritorial sectorization of potato (S. tuberosum) cultivation in the state of Carabobo, Venezuela. 

Agricultural lands of the municipality of Valencia where the capital of the state of Carabobo (the city of Valencia) is located, have 65 km² of soils suitable for cultivating potato, with an estimated sowing date in the second decade of May in the southeast and to the north. Towards the south of the city of Valencia, planting dates range from the third decade of May to the first of August; in the south of the municipality, the dates are found between the second decade of June and the second decade of August. Meanwhile, in the rest of the suitable soils, potato can be sown between the first and third decades of July.

On the other hand, in this municipality, 15 km² of moderately suitable soils were identified. The areas classified as moderately suitable for potato cropping are located north of the city of Valencia in hilly and mountainous areas where between 90 % and 95 % of the yield can be obtained during crop harvest. The estimated sowing dates for moderately suitable soils to achieve maximum potato yields of 95 % range from the second decade of May in the south and north. Further, between the first decade of June and the second decade of July in localized areas north of the city of Valencia, the estimated sowing dates for potato are found, and between the third decade of July and the first decade of September towards the center and northern areas of the municipality.

In figure 3, the municipality of Guacara can be observed as having soils suitable for potato cultivation, predominating low areas with hills, mountains and the zone corresponding to the lake plain north of Lake Valencia, which represents approximately 59 km² of loamy and moderately drained soils. The sowing dates estimated for this region to obtain yields between 95 % and 100 % are found between the second and third decades of July to the east and northeast. Meanwhile, the rest of the suitable soils have a tentative sowing date for the third decade of July.

Regarding the moderately suitable soils for potato cultivation in the municipality of Guacara, an area of 3 km² was identified with sowing dates estimated for the third decade of July, considering a maximum yield of less than 95 %. The above is the result of the light texture and the excessive drainage that these soils have, and therefore, the necessary humidity during the critical cultivation phases for potato is much lower than in soils of medium textures.

On the other hand, in approximately 122 km² of the Carlos Arvelo municipality, planting dates were estimated for the second decade of May and the first decade of June, with the possibility of obtaining 90% to 95% yields during the harvest. This is because soils are moderately suitable for cultivating potatoes due to their edaphoclimatic conditions and because the water availability for critical crop phases ranges between 315 and 275 mm.

The municipality of Bejuma has an area of 23 km² with soils suitable for growing potato, mainly in the area of the Chirgua Valley in the center of the region. The estimated sowing date for these zones is found from the second decade of May to the second decade of June, meanwhile, around the town of Bejuma, the dates were estimated between the third decade of May and the third of June. In the high areas of the valley, the sowing dates coincide between the first decade of June and the third of July. Also, the Chirgua sector is recognized as an important potato cultivation area in the country, due to the technification conditions of farmers.

Likewise, an area of 19 km² of moderately suitable soils for potato cultivation in this municipality was found located in hilly and mountainous areas, with slopes between 6 % and 10 %. These areas have soils with sandy textures and excessively drained that reach a maximum yield of 95 %, and sowing dates are found between the first decade of July and the third of August. Water availability in these lands of moderate suitability shows values between 315 and 275 mm during the critical phase of the crop.

Potato yield values achieved in the study area are due to the genetic makeup of the cultivar used and the environmental factors of the region. In this sense, the temperature and the number of light hours represent the environmental factors of greater weight in the development cycle of a crop, without neglecting the vital contribution of soil moisture variations due to rainfall (^{Kooman, Fahem, Tegera, & Haverkort, 1996}; ^{Pereira, Villanova, Ramos, & Pereira, 2008}; ^{Quintero et al., 2009}; ^{Streck, Matielo de Paula, Bisognin, Heldwein, & Dellai, 2006}).

In this regard, Spitters and Schapendonk (1990) established that there are potato varieties tolerant to drought; these keep their stomata open under severe water deficit conditions, so they maintain photosynthesis and the tissue damage of the leaves is lower, generating high yields.

According to Morales et al. (2015a), the high tolerance to drought of some potato varieties is mainly due to favorable anatomical and physiological characteristics to adapt to stress conditions, including lower stomata density, smaller stomata, a minimal decrease in the relative water content, and greater stability of the cell membrane.

Nonetheless, it is possible to resort to irrigation in dry seasons or during drought events to cover the water needs of the crop to maintain a constant production (^{Olivares & Zingaretti, 2018}). However, there are areas in Carabobo where irrigation systems are scarce or null due to some factors that condition its use, such as inaccessibility to nearby water sources, lack of water service, and absence of infrastructure, among others.

Most of the potato production systems in the state of Carabobo are characterized by a dependence on imported seed, an insufficiency of certified seed, the use of traditional varieties with low yield, and high susceptibility to diseases ( ²⁰; ^{Olivares et al., 2017}). Additionally, in some areas, there is inadequate use of water resources for irrigation, excessive use of pesticides, and incorrect management of chemical and organic fertilization practices.