Subsurface Porous Pipe Irrigation with Vertical Option as a Suitable Irrigation Method for Light Soils

MATERIALS AND METHODS

Experimental Site
Sand dunes in Khuzestan Province are spread between eastern longitude of 47° 40’ to 49° 20’ and northern latitude of 31° 5’ to 32° 20’ and these hills are extended from North West to South East. Total area is 0.35 million h, which represent 5.4% of total area of Khuzestan and 29.5% of agricultural lands. The field study was done in October 2003 at the Albaji Natural Resources Research Station (lat. 31° 20’ N and long. 48° 40’ E and 20 m asl). Average soil bulk density from 60 cm soil profile at the site is around 1.55 g cm^-3. Average initial water content of dry soil, average field capacity and average saturated condition from 90 cm soil profile at the site are around 1.5, 15 and 25% (w/w). Soil textures of the experimental site from depths of 0-30 and 30-60 cm are very similar. The average soil texture is categorized as sand and its particle size analysis is presented in Table 1.

System Structure and Installation
For this current study, porous pipe (type HD2216) with 22 mm external diameter was used. Three sets of porous pipe (each set included five the same sections) with three different lengths of 30, 45 and 60 cm were provided. To permit emitting of water only through the porous of pipe, end of the pipes were blocked by small caps. To consider uniformity of tests, each set included sections of unused and dry pipes. Then, each prepared set of porous pipes with the same length were connected to a polyethylene pipe of 32 mm diameter by fitted connectors considering 1.5 m distance between each pair of porous pipe sections to prevent interaction of soil moisture patterns resulting from porous pipe discharges.

Table 1:	The average soil texture of Albaji hills from a soil profile of 60 cm depth

Fig. 1:	Schematic structure of system installation

Fig. 2:	Schematic observation of soil moisture pattern and sample points

Immediately after each connector, a small control valve was installed on each porous pipe section because each section was considered to irrigate only each defined duration according to its order on polyethylene pipe. For example, the first section had to irrigate 10 min, the second 30 min and the fifth section (final one), 300 min. Therefore, five sections with the same length of porous pipes were ready to intake water from polyethylene pipe for delivering it inside soil as an emitter. One end of the polyethylene pipe was blocked and the other end linked to a Robin pump that pumped the required water from a tank to the system. A control value and a gauge regulated the required pressure head. Finally, all sections of each set of porous pipes were fitted in a set of prepared holes, which were dug out vertically. This system was made for mentioned three different lengths of porous pipes and repeated for application of 2, 4 and 6 m of water pressure heads (Fig. 1 ).

After preparation of system, tests were done over each set (including five sections of porous pipe with the same length) so that after 10 min of irrigation, the first valve on the first section was blocked and irrigation was terminated. Then by making a vertical cross section along the porous pipe, the produced symmetrical soil moisture pattern was observed (Fig. 2) and recorded. At the same time, 10 soil samples were collected from each cross section for laboratory measurement of soil moisture content. The locations of collected soil samples are shown in Fig. 2. This method of observation was repeated for other porous pipes after 30, 60, 180 and 300 min. By this methodology, it was possible to put all soil moisture patterns and lines over each other on one sheet specified for each applied water pressure head. Therefore, all soil moisture movement were observed with time for time periods from 10 to 300 min. Due to application of three lengths of porous pipes, three pressure heads and five irrigation durations, 45 soil moisture patterns were collected. From this large database, data of soil moisture patterns and lines resulting from 300 min of irrigation have sited in results section. To make these data simple to understand, they are shown in the form of diagrams as well. To predict the value of vertical and horizontal soil moisture movements with time, some empirical functions were fitted to the collected data.

RESULTS AND DISCUSSION

Vertical Soil Moisture Expansion
Results of soil moisture movement patterns for porous pipes of 30, 45 and 60 cm lengths are shown in Table 2 by y parameter and are plotted in Fig. 3-5, respectively. To predict the value of vertical soil moisture movement with time, some empirical functions were fitted to the collected data. These functions are shown in Table 3. Figure 3 shows that for porous pipe of 30 cm length with 300 min. irrigation duration with 2, 4 and 6 m of water pressure head, vertical expansions of soil moisture are 45, 50 and 60 cm, respectively. These expansions of soil moisture for porous pipe of 45 cm length (Fig. 4) are 60, 65 and 85 cm when the same irrigation duration and water pressure head were maintained. In case of 60 cm length of porous pipe under same irrigation duration and water pressure head, vertical expansion of soil moisture was, 78, 85 and 110 cm, respectively (Fig. 5). However, for all three porous pipes with different lengths, maximum vertical expansion of soil moisture occurred when, maximum water pressure head of 6 m was applied. Application of this highest water pressure head, resulted in to maximum vertical expansion of soil moisture, which was approximately equal to the length of porous pipe. In other words, at 6 m water pressure head, total vertical expansion of soil moisture was approximately equal to double the pipe’s length. For simplifying these results to users, empirical equations were extracted from the data of soil moisture movements with time (Table 3). As past to produce Y. Value of constant parameter shows pipe’s length for all equations. As shown, all equations are simple and linear with Y (cm), defined as vertical soil moisture from the soil surface to the end of moisture pattern and t (min), as time. Coefficients of determination, R² for all fitted equations is around 0.9. Therefore, these equations have been well fitted to the data and vertical soil moisture patterns should be well predicted by these equations. Because of these, user could be able to lead irrigation water down conformed to root zoon, or lead root system downward following produced moisture pattern. Therefore, one of the aims of current study might be achieved.

Fig. 3:	Soil moisture patterns for porous pipe of 30 cm

Fig. 4:	Soil moisture patterns for porous pipe of 45 cm

Fig. 5:	Soil moisture patterns for porous pipe of 60 cm

This is an advantage over other irrigation systems for sand soils, because of reducing evaporation from soil surface and preventing deep percolation and also preserving tree failure against wind by leading root system enough down. These results are significant and unique findings of the current research.

Horizontal Soil Moisture Expansion
Horizontal expansion of soil moisture patterns for porous pipes of 30, 45 and 60 cm lengths under 2, 4 and 6 m of water pressure head and irrigation duration of 300 min are shown in Fig. 3-5, respectively. Also, for predicting value of horizontal expansion of soil moisture with time, some empirical functions were fitted to the recorded data. These functions with best fitness to logarithm of observed data are linear as controlled by coefficient of determination R² and are shown in Table 4-6. These functions were extracted from data of horizons of 0, 15 and 30 cm for porous pipe of 30 cm, from horizons of 0, 30 and 45 cm for porous pipe of 45 cm and from horizons of 0, 30 and 60 cm for porous pipe of 60 cm. Shapes of all the remainder patterns under porous pipe ends are approximately conical. Therefore, these patterns are predictable by using last function for knowing radius of X at each horizon of porous pipe end and vertical soil moisture Y for the same time period of irrigation used for calculation of X. As shown in Fig. 3, for porous pipes of 30 cm length with 2 m of water pressure head, horizontal expansion of soil moisture varies between 5 to 12 cm; with 4 m of water pressure, it ranged between 5 to 15 cm and with 6 m of water pressure, it was 12 to 38 cm. For porous pipes of 45 cm length (Fig. 4) with 2 m of water pressure head, horizontal expansions was found between 6 to 12 cm; with 4 m of pressure head, it was between 6 to 16 cm and with 6 m of water pressure head, it was between 13 to 48 cm. For porous pipes of 60 cm length (Fig. 5), with 2 m of water pressure head, horizontal expansion was between 4 to 13 cm; with 4 m water pressure head it was between 5 to 16 cm and with 6 m of water pressure head it was between 20 to 60 cm. These data show that horizontal expansion of soil moisture varied a little by changing the length of the pipe. However, variations were much more when water pressure head changed from 2 and 4 to 6 m. Therefore, this system can be designed for porous pipe of 30, 45 and 60 cm lengths with at least 6 m of water pressure head because of not making a proper horizontal soil moisture pattern when 2 and 4 m of water pressure head of irrigation water was applied. The reason of limitation for horizontal expansion of soil moisture was due to soil texture, which was sand and irrigated water was more affected by gravity force than the matrix force.

Table 2:	Vertical (y) and horizontal (x1 and x2) expansion of soil moisture movements (cm) around porous pipes after 300 min of irrigation

Table 3:	Empirical equations fitted to vertical soil moisture movements as functions of time (Y in cm and t in min)

Table 4:	Empirical equations fitted to horizontal soil moisture movements as functions of time (X in cm and t in min) -porous pipe of 30 cm

Table 5:	Empirical equations fitted to horizontal soil moisture movements as functions of time (X in cm and t in min) -porous pipe of 45 cm

Table 6:	Empirical equations fitted to horizontal soil moisture movements as functions of time (X in cm and t in min) -porous pipe of 60 cm

Table 7:	Moisture content (W/W) at 10 points in the wetted profile around porous pipe after 300 min of irrigation

However, using 2 or more porous pipes together, might make the system able for applying 2 and 4 m of water pressure head because diameter of horizontal pattern of soil moisture will extend much more.

Actual Soil Moisture Content
Figure 6-14 depicted actual soil moisture content lines (w/w %) for porous pipes of 30, 45 and 60 cm lengths, in which irrigation was applied continuously for 300 min at 2, 4 and 6 m of water pressure heads.

Fig. 6:	Soil moisture line during application of 2 m water head

Fig. 7:	Soil moisture line during application of 4 m water head

Fig. 8:	Soil moisture line during application of 6 m water head

Fig. 9:	Soil moisture line during application of 2 m water head

Fig. 10:	Soil moisture line during application of 4 m water head

Fig. 11:	Soil moisture line during application of 6 m water head

The value of soil moisture decreased a little, when the distance of wetting front pattern increased from porous pipe location. This was because of decreasing matrix potential gradient from pipe location to wetting front. In addition, increasing pressure head from 2 to 4 and 6 m, showed a little increase in soil moisture content. However, minimum moisture content of 14.8% (w/w) for porous pipe of 30 cm length at 2 m of water pressure head and 17.4% (w/w) for porous pipe of 60 cm length at 6 m of water pressure head was recorded (Table 7).

Fig. 12:	Soil moisture line during application of 2 m water head

Fig. 13:	Soil moisture line during application of 4 m water head

Porous pipe with 2 and 4 m of water pressure does not make a proper horizontal moisture pattern as explained in previous section. However, these soil moistures are around average field capacity moisture content 15% (w/w) which was obtained from the experimental site and are conformed to results of Akhoond-Ali (2003) for the same soil and are proper moistures for plant to use. Therefore, from this point of view the objective of this study is satisfied because with field capacity condition, the lost water from deep percolation can be preserved.

Fig. 14:	Soil moisture line during application of 6 m water head

Attention to increasing soil moisture in a few percents with increasing water pressure, leads to this point that if the pressure head increases for levels more than 6 m, the produced soil moisture might increase toward higher levels up to saturated condition. Therefore, some tests for higher pressure heads are needed to generate a general empirical equation, related produced soil moisture to pressure head.

CONCLUSIONS

An initial test on a new idea of using porous pipe vertically was presented. This system was designed for three lengths of porous pipe (30, 45 and 60 cm) with three water heads of 2, 4 and 6 m and for a maximum irrigation duration of 300 min. The experimental results showed vertical expansion of soil moisture to the extent of 200% of porous pipe length in all treatments. However, acceptable horizontal expansion of soil moisture was recorded only for application of 6 m of water head. The produced soil moisture content by this system was shown in a range between 14.8 and 17.4% (w/w), which is around field capacity water content. Therefore, it can support plant growth in effective manner.

This system might have two advantages over horizontal installation of porous pipe. First, it might be able to lead root system down enough following length of porous pipe and downward emitted water to preserve tree failure against wind. The second advantage is involved in pipe obstruction by root system. In terms of horizontal installation of porous pipe, obstruction will interrupt all the line. However, obstruction in vertical installation will change the pattern of soil moisture only for the obstructed part, which is an off-line branch from lateral line and works locally like a dripper.