Expansión de los casos de usos del sistema

The agriculture sub-model comprises two groups: vegetables and fruits, which are representative of the main agriculture in the study area (INEGI, 2010). Vegetables include: tomato, maize, cucumber, habanero and green chilli, zucchini, avocado, and henequen. Fruits group includes: watermelon, orange, lemon, mamey, coco fruit, papaya, and other fruits. In order to estimate water usage and wastewater discharge from agriculture activity, it is important to consider fundamental principles based on FAO, (1986). Crop water utilization is defined as “the amount of water needed to meet the water loss through evapotranspiration (Eo), and to grow in optimal conditions”.

In order to derive crop water utilization, there are three environmental parameters that need to be considered: climate, crop type, and growth stage of the crop. The influence of the last two parameters is reported as crop factor (Kc).

- Climate influence on crop water needs (Eo) - it is given by the reference crop evapotranspiration (Eo) used by FAO (grass), expressed in millimetres per unit of time (i.e. mm/day; mm/month; mm/season). The rate of Eo is determinate from a large area, by green grass 8 to 15cm tall, which grows completely in shades and with no shortage of water; either by experimental evaporation (such as the pan method) or by measured climate data (such as the Blaney-Criddle method).

- Crop type influence on the crop water needs (Kc) – it depends on three factors: the type of crop, the growth stage of the crop and the climate. In order to determinate Kc, there are three steps: 1. Determine total growing period of each crop is needed;

2. Determine the various growth stages of each crop; and 3. Determine the Kc value for each crop at each stage of growth or ETo (estimated above). Typical values of total growing period and growth stages for various crops are documented by FAO, (1986). All these values need to be considered.

Table 45 Water utilization efficiency per crop (kg/m³)

* Vegetables and fruits used to estimate average for each type of crop. Maize was estimated with its original value.

From the above explained, FAO, (2013) have reported the water utilization efficiency for harvested yield (Ey) for specific crops (Table 45). Water utilization efficiency (Y/ET) is defined as yield of plant product (tonnes of crop, Y) per unit of crop water use (litres of water lost by evapotranspiration, ET) (Atwell et al., 1999). Table 46 shows agriculture production by crop type (SAGARPA, 2010).

Table 46 Total crop production (m³/s) in Yucatan in 2005

Crops Type Total production (ton)

A B C D

Tomato vegetables 0.0E+00 4.5E+02 1.5E+03 1.1E+03

Maize vegetables 7.1E+03 1.0E+03 0.0E+00 0.0E+00

Cucumber vegetables 8.4E+01 3.0E+02 3.4E+02 3.4E+02

Habanero chilli vegetables 0.0E+00 2.6E+02 3.9E+02 2.4E+02

Green chilli vegetables 6.3E+01 0.0E+00 4.0E+01 0.0E+00

Zucchini vegetables 0.0E+00 4.1E+02 6.4E+02 4.4E+02

Vegetables vegetables 0.0E+00 3.7E+02 4.3E+02 2.9E+02

Avocado vegetables 0.0E+00 2.7E+02 4.8E+02 3.2E+02

Henequen vegetables 1.2E+02 8.3E+02 0.0E+00 0.0E+00

In order to derive the initial data input for the model, which starts simulating in 1990, agriculture production growth was assumed to follow population growth (average population growth rate of 1.74%/y). Then agriculture production was extrapolated back to 1990, and the results are given in Table 47.Table 48 is grouping crop production

from Table 46 into the three groups: vegetables, fruits and maize. Maize was classified separately since it is a major crop in the area but its value for water utilization efficiency is much lower than the vegetable value.

Table 47 Agriculture production (Tons), 1990

Crops Type Total production

A B C D

Tomato vegetables 0.0E+00 3.5E+02 1.2E+03 8.7E+02 Maize vegetables 5.5E+03 7.8E+02 0.0E+00 0.0E+00 Cucumber vegetables 6.5E+01 2.3E+02 2.6E+02 2.6E+02 Chile habanero vegetables 0.0E+00 2.0E+02 3.0E+02 1.8E+02 Chile verde vegetables 4.8E+01 0.0E+00 3.1E+01 0.0E+00 Zucchini vegetables 0.0E+00 3.1E+02 4.9E+02 3.4E+02 Vegetables vegetables 0.0E+00 2.8E+02 3.3E+02 2.2E+02 Avocado vegetables 0.0E+00 2.1E+02 3.7E+02 2.5E+02 Henequen vegetables 9.6E+01 6.4E+02 0.0E+00 0.0E+00 Watermelon fruit 4.9E+02 5.2E+02 2.3E+02 2.3E+02

Orange fruit 0.0E+00 2.1E+03 2.2E+03 0.0E+00

Lemon fruit 0.0E+00 1.9E+02 3.0E+02 0.0E+00

Mamey fruit 0.0E+00 1.7E+02 1.7E+02 1.7E+02

Coco fruit fruit 0.0E+00 1.4E+02 2.9E+02 2.5E+03

Citrus fruit 2.6E+02 1.6E+03 1.5E+03 1.2E+03

Other fruits fruit 0.0E+00 8.5E+02 1.2E+03 9.7E+02

Papaya fruit 0.0E+00 4.2E+02 5.8E+02 4.2E+02

Total fruits fruit 7.5E+02 5.9E+03 6.5E+03 5.5E+03 Total Veg vegetables 5.7E+03 3.0E+03 3.0E+03 2.1E+03 Total Production Veg + Fruit 6.4E+03 8.9E+03 9.5E+03 7.6E+03

* Total veg is estimates without maize. Total Production includes: vegetables, fruits, and maize.

Table 48 Total agriculture production (Tons), 1990

Crops A B C D Total

production Vegetables 2.1E+02 2.2E+03 3.0E+03 2.1E+03 7.5E+03 Fruit 7.5E+02 5.9E+03 6.5E+03 5.5E+03 1.9E+04 Maize 5.5E+03 7.8E+02 0.0E+00 0.0E+00 6.3E+03 Total 6.4E+03 8.9E+03 9.5E+03 7.6E+03 3.2E+04

In 2013, Yucatan government reported a total water usage in the Metropolitan Area of Merida (MAM) for agriculture of 3.71x10⁷ m³/y (1.2 m³/s), which corresponds to a total area of 480 km² (Yucatan Government, 2013). The total agriculture area of MAM covered by the four sections of this study is 595 km². Therefore the total water usage for agriculture corresponding to the four sections was extrapolated to 1.5 m³/s in 2013.

The wastewater estimation was obtained considering the following assumptions:

- Water needed for the crop (water utilization by crop, Table 45) is less than the water irrigation volume reported, therefore, the remaining of the irrigation volume in Yucatan is considered as wastewater discharged to the aquifer (irrigation – crop water utilisation = wastewater).

- As there is no wastewater treatment infrastructure placed in agriculture fields, total wastewater reported will be considered untreated, which means the total load of pollutants (chemical and microbial) will reach the aquifer.

By combining the data of agriculture production per crop type in 1990 (Table 47) with the data of water utilization per crop type (Table 45), the water utilization per crop type and aquifer section were estimated and reported in Table 49, with a total crop water utilization of 0.29 m³/s in 1990 for the study area.

Table 49 Water utilization per crops (m³/s), 1990

Crops A B C D Total

Vegetables 6.9E-04 7.3E-03 9.8E-03 7.0E-03 2.5E-02 Fruit 4.1E-03 3.2E-02 3.6E-02 3.0E-02 1.0E-01 Maize 1.4E-01 2.1E-02 0.0E+00 0.0E+00 1.7E-01 Total 1.5E-01 6.0E-02 4.5E-02 3.7E-02 2.9E-01

By extrapolating back based on population growth the total irrigation water use for agriculture from 2013 to the year 1990, a value 0.79 m³/s is obtained for the study area (Table 50).

The corresponding wastewater discharge, as given in Table 51, is derived by

“subtracting” Irrigation – crop water utilization = Wastewater. The total wastewater discharge in the study corresponds to 0.5 m³/s, i.e. 63% of the irrigation water usage.

Table 50 Water use in agricultural Irrigation (m³/s), 1990

Crops A B C D Total

Vegetables 1.9E-03 2.0E-02 2.6E-02 1.9E-02 6.7E-02 Fruit 1.1E-02 8.7E-02 9.6E-02 8.1E-02 2.8E-01 Maize 3.9E-01 5.6E-02 0.0E+00 0.0E+00 4.5E-01 Total 4.0E-01 1.6E-01 1.2E-01 1.0E-01 7.9E-01

Table 51 Wastewater discharge from agriculture (m³/s), 1990

Crops A B C D Total

Vegetables 1.17E-03 1.25E-02 1.67E-02 1.19E-02 4.22E-02 Fruit 6.94E-03 5.51E-02 6.07E-02 5.10E-02 1.74E-01 Maize 2.47E-01 3.50E-02 0.00E+00 0.00E+00 2.82E-01

Total 2.5E-01 1.0E-01 7.7E-02 6.3E-02 5.0E-01