Technical Efficiency of the Subsurface Drainage on Agricultural Lands in the Moldova River Meadow

This study aims to investigate the technical efficiency of different subsurface drainage variants, in terms of the depth of the tile drains, spacing between the drain lines, type and thickness of the drain + filter complex, and the improvement procedures. Within the four variants, the discharge rate of the soil moisture excess was studied. In variants A and D, the spacing between drains is 20 m, and in the variants B and E, the spacing is 15 m. The depth of the tile drains is 0.8 m in variants D and E and 1.0 m in variants A and B. In variant A, tile drainage was combined with land shaping in the bedding system with top of ridges and furrows. Soil moisture was determined on checkpoints placed on drain cross section, at 2 m from drain lines, and of the middle of the drain spacing. In the version with land shaping, the drain lines located under the furrows favor the excess moisture removal. A similar technical efficiency was recorded in unimproved variant but with spacing between drains of 15 m. Best efficiency at removing excess water was registered in variant of the filtering material from ballast associated with flax strains.


Introduction
Soil quality is more or less affected by one or more conditions, such as drought, periodic excess water, soil erosion, landslides, etc. [1,2]. Their harmful influences are reflected by the degradation of soil characteristics and functions, of soil bio-productive capacity, affecting yields and food safety, with consequences on human life quality [3][4][5]. These conditions are determined either by natural factors [6][7][8] or by agricultural and industrial activities that might have a negative synergic action [9,10]. Some of the main limiting factors of crop production in Suceava County are excess water, floods, low infiltration rate and soil compaction, soil erosion, and landslides. Waterlogging is a complex process determined by water supply, retention, movement, and discharge in the sub-adjacent rock-soil system [11,12]. Water excess (rainfall water and/or groundwater) gets manifest under different forms and intensities, both in case of flat and slope lands [13][14][15]. Natural conditions of the Baia Piedmont Plain favor the occurrence and maintenance of waterlogging. The Moldova River meadow and terraces platform in the form of stripes with an average width of 1.5 km, almost parallel to Moldova River bed, oriented from North West to South East, of gentle slope (1-5%), with flat-horizontal areas and a lot of micro-depressions, facilitating water storage. Because of the humid climate in the Radaseni-Fantana Mare-Baia depression, at low evapotranspiration, the 1-5 days of consecutive heavy rainfall are the main source of water excess in the poorly permeable soils. Furthermore, the hydrographic network represented by the Somuzul Baii runlet, which crosses the mid-area of Radaseni-Fantana Mare-Baia, with a riverbed of about 0.3-0.6 m depth and shallow at some sectors, is a permanent cause of excessive humidity, prone to flooding nearby agricultural lands during heavy rains. Groundwater is stored in permeable deposits of sands with gravel and rocks, and its level is generally free, yet in meadow and low terrace areas, at highest levels, it flows under pressure, causing rising of the water table.
Experimental drainage fields were organized to explore the wide range of causes conducive to excessive water, the complexity of natural conditions hosting the phenomenon, and the multitude of its forms of manifestation. Study results are the base for solutions applied in implementing planning projects of the said areas and, by extrapolation, they were the basic materials for spreading technical solutions in natural homogenous conditions, concretized in the elaboration of design and use standards, instructions, or recommendations.

Material and method
Based on the pedo-climatic conditions of the wet area of Suceava County (Figure 1) and from the humid area of the meadow and terrace platform of the Moldova, respectively, subsurface drainage experimental fields were organized as a main mean of rainfall excess removal, locally associated to different land improvement works.
The pilot experimental agricultural drainage field of Baia was organized in 1978 by the National Society for Land Improvements-Suceava branch-in collaboration with the Agronomical University of Iasi, on a pseudo-gley glosic albic luvisol, used in natural pastures and hayfields with periodic excessive humidity from precipitations.
The hydro-technical scheme of the Baia experimental drainage field (Figure 2) comprises a surface of 3.00 ha, organized by subdivided lots, with two repetitions of three variants each, distinguished by distance between drain lines (12; 15; 20 m), average depth of the pipe drains is 0.8-1.0 m, type and diameter of drain pipes, type and thickness of filtering materials ( Table 1).   To improve the production capacities of agricultural lands and mostly of farm fields, surface and subsurface drainage system, banking, watercourse regulation, soil erosion control, and other such works were conducted. According to data from the National Association for Land Improvement, in Suceava County, there is an area of 44,904 ha with surface drainage works, of which 27,455 ha were completed with underground drainage works. The drainage ditches are of 1875 km in length, while the subsurface drainage network has a total length of 11,909 km.
Based on results from the Baia agricultural drainages experimental field, technical solutions for the Moldova River meadow depletion-drainage planning were established. Rotopanesti-Radaseni-Fantana Mare, Dragoiesti-Berchisesti, Bogdanesti-Baia, and Baisesti-Dumbrava surface and subsurface drainage systems comprise a total surface of 8761 ha, of which underground drainage covered an area of 3059 ha (Figure 3).
The surface drainage ditch network with a total length of 126.85 km is made of collecting, disposal and interception channels, and so on. To discharge excessive soil moisture, an underground drainage network was organized, made of tile drains with a total length of 1575.12 km, based on the nature and intensity of excessive water.
The Baia agricultural experimental drainage field is placed in the northern part of the Rotopanesti-Radaseni-Fantana Mare depletion-drainage system, on a platform of the upper terrace, on the left side of the Moldova River, located North West from the Baia depression.
In the natural conditions of this wetland, the albic stagnant-lossic luvisol is used as a natural pasture. The surface and subsurface drainage systems, as a measure of moisture control, were associated to works of land shaping in the bedding system (Figure 4), deep soil loosening, mole drainage, amendments, and so on. After implementing improvement procedures, their operation and behavior are of high importance.

Results and discussions
The analysis of soil moisture values at 10 cm depth, 2 days after cumulative rainfall of 40 mm, on the variants with the spacing between drains of 15 m, shows that in case of control points placed 2.0 m from the drain line and at mid-distance between drains, the moisture level got to the depth of 30-40 cm, where the poorer permeable layer is located and decreased ( Figure 5). As for D 3 tile (distance between drain lines 20 m and depth of the pipe drains 1.0 m), with the land shaping in the bedding system with top of ridges and furrows, the soil moisture at control points located 2.0 m from the drain line and mid-distance between drains (10.0 m) got to the depth of 40-50 cm, due to the harsher material placed when the ridge-and-furrow system was created, which implicitly increased the depth of the poorer permeable layer (Figure 6). In all cases, at the control point located on the drain line, soil moisture values 2 days after rainfall generally increase as depth increases, as the water flow is directed to the drain line and the permeability of the filtering layer got poorer over the 36-year operating period.

Current Perspective on Irrigation and Drainage
After 5 days from rainfall, soil moisture at control points of D 5 , D 12 , D 13 , D 14 , and D 15 drains located at 2.0 m from the drain line and the middle of the drain spacing generally increases from a 10-20 cm depth to a 40-50 cm depth; then it slightly decreases because of the poorer permeable layer (Figure 7). Higher moisture in the superficial soil layer is due to water storage by organic material from the discontinuous celery layer, as the drained surface is used as pasture.    At D 3 drain with ridge-and-furrow system, soil moisture values have the same dynamics; the only difference being that the depth where the greatest moisture is recorded increases from 50 to 60 cm for the control point located at 2.0 m from the drainage ditch to 60-70 cm mid-distance between drains, due to water excess elimination and water consumption by plants (Figure 8).
At the drain line, soil moisture values generally decrease to the depth of 20-30 cm, and then they increase thanks to the water flow directed to the drain filter during the operating period. For the first 20 cm, moisture values are higher because of the plant root system, which is better developed on the drain cross section.
The analysis of the values of the average content of soil water by control points, established 3 days from rainfalls, shows that at the drains placed at 15 m one from another, the lowest value is recorded at the control point located 2.0 m from the drain line, while the highest value is recorded on the drainage cross section (Figure 9).  At D 3 drain with distance between drain lines of 20 m and with ridges and furrow, the highest value is also registered on the drainage ditch, but the lowest value was obtained at the control point located mid-distance between drain lines, due to the water flow directed to the drain filter and runoffs toward the drain line produced during heavy rainfalls (Figure 10).
Runoff control toward the drain line in case of D 3 drain with a ridge-and-furrow land surface is also shown in the analysis of the average soil moisture recorded at D 12 drain, with the distance between drain lines of 20 m, but with the non-modeled land surface, where the highest value was obtained for the control point located mid-distance between drains (Figure 10).
After 5 days, the last precipitations, the average moisture reaches the highest peak at the control point located mid-distance between drain lines, while the lowest value is registered for the drain cross section, except for D 3 drain (Figures 11 and 12). The decreasing values of average soil moisture water at mid-distance between drains to the drain line show drain system operation after 36 years of activity.
For the D 3 drain with ridges and furrow, the lowest value of the average moisture is also recorded at the drain line, yet the value obtained at the control point mid-distance between drains (on ridge) is 1.77% lower than the value at the control point 2.0 m away from the drain line, due to any runoff control to the drain line and to having added harsher material when the ridges were created. For such material, hydro-physical indices (fading coefficient, field water capacity) are lower than those of finer textured material (with high clay content).
The analysis of soil moisture content in the control section, 2 days and 5 days from the rainfall recording, of the studied drains (Figure 13), shows that the efficiency of water excess removal may be assessed by using different drainage technical solutions after 36 years of operation.   At D 13 drain with a filtering layer made of 30 cm thick flax stalks, after 36 years of operation, they turned into organic matter, prone to poorer permeability around the drain pipe and water flow to the drain, recording 2 days from rain the highest average soil moisture at the control section of 31.85% and 5 days after the value of 29.48%. At the same drain, 2 days after the last rainfall, the greatest amplitude of the soil moisture of 4.39% per control points was achieved.
Maintaining the distance of 15 m between drains, the same filtering material (ballast layer of 12 cm thick and flax stalk layer of 20 cm thick) and increasing the laying depth from 0.8 m (D 14 ) to 1.0 m as for D 5    By increasing the laying depth to 1.0 m, maintaining the distance of 20 m between drain lines and land shaping in the bedding system with top of ridges and furrow as in D 3 drain variant, a better discharge of excessive water is achieved, yielding an average soil moisture of 30.11%, respectively, and of 28.97% for the control section, values that are relatively close to those determined in case of the drains placed every 15 m, except the D 13 drain.

Conclusions
• In case of subsurface drainage systems from the Moldova River meadow, the predominant spacing between drains is 20 m, and the depth of the tile drains is 1.0 m in the terrace area.
• For an efficient water excess removal, it is advisable improvement procedure as land shaping in the bedding system with ridges and furrows in order to ensure a good drainage.
• Ridge-and-furrow system favors the discharge of water excess, as the runoffs are led to drain lines, obtaining a better water collection and discharge during the first hours and days from excessive water incidence.
• The average soil moisture content of 2 days following the rainfall has the smallest value close to the drain lines, due to the water afflux toward the drains and of the decrease of the permeability of the filtering layer, in time.
• The use of the flax stalks only as filtering material, regardless of the thickness of the layer, is not recommended because of permeability diminishing. However, it was observed that their association with ballast ensures the best discharge of the moisture excess even after 35 years of operation.