Study and design of an agricultural drainage in “La Trapa farm”, Maule region, Chile.

Cristián Kremer; CE-547; UNM; 2020.

Introduction

Walnuts production has been an attractive business since 2006 in Chile. In this framework the surface planted has increased more than 50 % in the last ten years. However the best suitable soils to develop this fruit tree are already occupied, which has forced new investors and farmers to look for lower quality soils to develop it. One of the main issues with walnuts production is a disease called “Phytoptora” which is a fungus that attack roots and stem of the tree causing its death (Figure 1). Usually this disease is associated to humid environments near these tree structures. In this context in 2016 the engineering consultant office “Agritek s.a.” was mandated to develop a study and design of an agricultural drainage in “La Trapa” farm. The main objectives of this study were, to do a soil characterization of the farm to determine the main limitations causing the condition of restricted drainage; to identify the main sources of recharge of the water table; and design and built an agricultural drainage system suitable for the production of walnuts.

Figure 1. Picture of a walnut tree attacked by “Phytoptora”

Materials and Methods.

Project Location: The study was conducted in “La Trapa” farm located in the Maule Region, Chile, South America (Lat: 36° 00´ 18.05´, Long: 71° 45´ 10.57´´). “La Trapa” farm is placed at 331 km south from the Capital Santiago (Figure 2) and it has a surface of 60.7 ha (Figure 2).

Figure 2. “La Trapa” farm location

Projection: Chile lies in UTM zone between 18s and 19s (Figure 2). Due its North-South longitude (4270 km) and its East-West longitude (max: 440 km, min: 90 km), most of the public institutions in Chile which have survey available information such as the “Instituto Geográfico Militar” or the Forest Service are using this projection because it looks more suitable for Chile´s geography. Additionally, the Topography map that was initially available from the field was referenced in UTM coordinates, because of these reasons, a UTM projection was used with the following features: Zone:19s; False-Easting: 500000; False-Northing: 10000000; Central-Meridian: -69.0; Scale-Factor: 0.9996; Unit: Meters. The Geographic co-ordinate system used was WGS-1984.

Data collection: Most of the data used in this work was collected in- situ and projected later in an Arc-Map environment. Soil description was made through 64 pit holes evenly distributed on the field (Figure 3a), using the Soil Survey Taxonomy Manual (USDA-NRCS), soil hydraulic conductivity (K) was measured in 21 auger holes with the presence of water table using the Hoodhougt method (Figure 3b). Water table variations were monitored for three years (2016-2018) through 21 piezometers installed in the same auger holes previously used to measure K (Figure 3b). There was an original topography map in Autocad which was transformed to an Arc-Map environment (Figure 3). As a base imagen, a KMZ file for the “La Trapa” farm was transformed an used on an Arc-Map environment. Data was ordered in tables to be later processed and viewed in an Arc-Map environment, different vector and raster features were created. For the final design, maximum rainfall data for 1,2,3 days with a 5 days return period were used to build a DDF curve, which helped together with the soil water holding capacity data to determine the amount of water percolated. A modified Darcy´s equation was used to determine the amount of water harvested for each drain pipe, and its separation was determined using the model proposed by Glover and Dumm for a discontinuous recharge over an impervious layer.

(a) (b)

Figure 3. Surface contour lines and (a) pit holes locations, green dots (b) piezometers location, red dots.

Data Analysis: The Spline interpolation was used to create surfaces of the clay content percentage, soil water holding capacity (SWHC), water table variations, and soil effective depth, It was used with a tension type which helped to tune the stiffness of the interpolant according to type of the data used. Also water table contour lines were created. On the other hand, to create a surface of soil hydraulic conductivity the IDW interpolation method was used because it looked like more suitable due the level of heterogeneity of the data, then the main point had more influence in the interpolation than the more distance ones. To get the percentage and value of surface which was larger or shorter a specific value, the raster calculator was used to create new raster surfaces showing these queries. Some features were transformed from vector to raster to use the raster calculator and also from line to polygon or vice versa to perform geoprocessing.

Results

Figure 4a shows the clay content percentage on the first 0.7 m depth of the soil. This percentage ranged from 26 to 38%, where the purple colored bands represent the largest values (32 to 38%) and the brown colors the lowest (26 to 32%). Texture wise, most of the field was classified as Clay Loam but a small area on the south west was classified as a Sandy clay loam. The SWHC was high varying from 10 to 15 cmm^-1. Most of the surface was in the range of 12 to 14 cmm^-1(Figure 4b).

(a) (b)

Figure 4. (a) Clay percentage content first 0.7m soil depth, (b) Soil water holding capacity cmm^-1.

The effective soil depth (EFS) was define by the occurrence of a massive horizon, without structure, low porosity and impervious. Figure 5a shows that EFS ranged between 0.9 to 2.1 m being the surfaces with red and orange colors the shallowest (0.9 to 1.3m) and the light blue and blue surfaces the deepest (1.5 to 2.1 m). Near the 50 % of the surface is in the range between 1.3 to 1.5 which is yellow colored (Figure 5b,c). In term of the design the target depth of the drain pipes was chosen to be 1.3 m because near the 70% of the surface has an effective depth larger than 1.3 m (Figure 5c).

(a) (b)

(c)

Figure 5. Effective soil depth. (a) effective soil depth ranges, (b) Percentage of the surface with effective depth deeper and shallower than 1.3 m., (c) Percentage of the total surface vs Effective soil depth ranges.

Soil hydraulic conductivity showed a wide range of values (Figure 6a), the red colors show the surface occupying for the lowest values ranged from 2.9 to 4.9 m day^-1, while the yellow colors show the middle ranged values from 4.9 to 6 m day^-1 and the green are the highest ranged values from 6 to 8.2 m day^-1. For the drainage system a K value which allows to get at least 87% of the surface well drained was chosen. On Figure 6b is possible to see that the value which fulfilled that requirement was 4.9 m day^-1.

(a) (b)

Figure 6. (a) Soil hydraulic conductivity (K) ranges, (b) Percentage of the total area with K lager and shorter than 4.9 mday^-1.

Soil was classified as an ALFISOL, and belong to the “Parral” Series. This are sedimentary soils, derived from volcanic tuff, in position of remaining terrace. It is characterized by presenting medium to fine textures classes originating a sequence of four to seven horizons of sandy clay loam, to clay loam in depth. Its drainage class varies from moderate to imperfect (CIREN-1997). Five sub orders were described (Figure 7), with variation in slope, Clay %, effective soil depth and drainage class.

Figure 7. Soil Map. PRL: “Parral” serie. Sub series: PRL-10; PRL-11; PRL12; PRL-13; PRL-14;PRL-15.

Figures 8 a,b,c show the water table elevation surface and water table contour lines, for a winter day in a particular year. No presence of free water was found in summer and fall. Therefore, the main source of water recharge was rainfall, and it is concentrated in Winter. Water table elevations and contour lines were pretty similar for the three years, so flow direction indicated with the blue arrows was quite conservative, which was further relevant to define the position of the relief drains.

Figure 8. Water table surfaces and water table contour lines, for a winter day for each year of measurement. Blue arrows indicate flow direction.

Conclusion

Based on the field data and its subsequent analysis, a drainage system was designed (Figure 9). The system is suitable for maintaining walnut production and can operate with a 48mm day^-1 percolation recharge. It is a relief drainage system, with rain being the main recharge of the water table that is seasonal (winter). The distance and depth of the drains was estimated to be 100 and 1.3 m respectively, and is designed to maintain a depth of free water in the soil of 1 m. The drains were distributed in the field as perpendicular as possible to the water table flow.

Future work

Just before leaving Chile, the construction of the system was almost finished. The next step is to monitor the behavior of the system and verify if the assumptions made to design it work well.

Figure 9. Final Layout of the drainage system ; collecting pipe; drain pipe ; water table contour lines (06/08/2016); observation chambers

Literature Review

CIREN. 1997. Descripciones de suelos, materiales y símbolos. Estudio agrológico VII región. Publicación CIREN N° 117, p:659.

Salgado L.1999. Manual de estándares técnicos y económicos para obras de drenaje. Ministerio de Agricultura, Comisión Nacional de Riego (CHILE), p:314.

Schoeneberger P., Wysocki D., Benham E., Broderson, W. 2002. Field book for describing and sampling soils, (ver. 2.0). Natural Resources Conservation Service, USDA, National Soil Survey Center, Lincoln, NE.