ABSTRACT: The increasing demand for sustainable food and non-food biomass production is challenging farmers all over the world, particularly in those areas where are short in water supply. Besides, the constant spreading of desertification in the Mediterranean region makes cropping even more difficult. In fact, the main problem related to desertification is the reduction of rainy events in a given region, although the annual rainfall remains constant. Hence, surface water reservoirs are accessible only for a limited period of the year forcing farmers to rely on belowground water, which is expensive and, sometimes, impossible to carry out. In the framework of MediOpuntia Project, a possible strategy to harvest rain water in arid and semiarid regions of the world, is represented by the installation of subsurface water retention Technology (SWRT) made of impermeable U-shaped barriers laid 80-100 cm belowground with aim to prevent water loss due to percolation. In addition, also soil nutrients loss is prevented as they will be collected by the membranes and kept available to roots after major rainy events. So far, the market still lacks the availability of machineries capable to lay impermeable membranes belowground in a single pass, and the present Project aims to fill this gap. The present study aims to develop and test a machinery capable to lay an impermeable membrane at 80-100 cm belowground to catch rainwater in arid and semi-arid areas.
1. INTRODUCTION
Global warming is one of the most important topic of the last years. Regarding this aspect, Mediterranean zone is worldwide considered a hotspot of global warming [1], indeed many studies highlighted how this part of the world could probably experience the highest drying trend worldwide [2,3].
The major part of climate trends studies reported a further drying scenario, particularly for North Africa, with decreasing rainfall and increasing average temperature [4,5].
On the other hand, studies focused on meteorological data showed a more complex description of the phenomenon.
Indeed, several studies highlighted increasing rainfall trend in North Africa with a climate which seems to switch to wetter conditions [6,7]. Therefore, the question which arises is “how to explain such controversial assertions?”.
The answer probably lies in the increasing number of extreme events in North Africa [8]. The key aspect of climate change in North Africa is indeed not the decreasing in precipitation, but the fact that rainfall is more and more concentrated in few extreme events alternating with longer drying periods [9,10]. This means that even in a year with high cumulative rainfall, there could be large periods of droughts, which could deplete water and food stocks [11].
These droughts could alternate with frequent extreme floods, which will accentuate human vulnerability by intensifying land degradation processes, loss of biodiversity, water availability and economic growth [12]. On the other hand, water from these events, when opportunely conveyed and treated, can represent a precious source of freshwater and this is properly the aim of water harvesting technologies [13]. In particular, considering the features of North Africa zone, an interesting approach consists of avoiding water losses through percolation, which in such zone is particularly accentuated thanks to the sandy texture of soils.
Retaining water in sandy soils, to allow for the efficient growing of a given crop, represents an age-old problem [14]. Scientific research has proposed various possible solutions, for instance the use of biochar or adsorbent polymers in the soil [15,16].
Among the various possible solutions Subsurface Water Retention Technology (SWRT) has shown interesting features [17]. SWRT consists of installing a waterproof membrane within the soil profile, in order to reduce the loss of water via deep percolation [18].
The base approach of SWRT is not something new, considering that methods to reduce water percolation in sandy soils adding an impermeable material have been investigated since ‘60s.
Various materials were tried for instance clay [19,20], gel conditioners [21,22], metal [23] or asphalt [24,25]. Among these materials, asphalt showed suitability to increase crop yield for what concerning bell pepper (Capsicum annuum L.), sweet corn (Zea mays L.), sweet potato [Ipomoea batatas (L.) Lam.] and rice (Oryza sativa L.) [26]. On the other hand, asphalt barrier are costly and labor intensive and their application is not sustainable [27]. In the last years, the development of polymer technology allowed switching from asphalt barriers to polyethylene membranes [28]. Interestingly, impermeable SWRT showed very positive results in several field trials.
Elawady et al. (2003) showed an 18% increase in spinach (Spinacia oleracea L.) yield [29], Awady et al. (2008) reported 141 to 190% increases in tomato (Solanum lycopersicum L.) yields [30], Kavdir et al. (2014) reported a 43% increase in vegetable production and a 238% increase in corn production on irrigated sandy soils after installation of polyethylene membranes [31]. Other studies performed in Iraq revealed a positive effect of SWRT impermeable film on both yield and water use efficiency in chili pepper (Capsicum annuum L.) [32,33].
Taking into account these interesting results, scientific research tried to deepen the knowledge on SWRT, identifying the most suitable geometry and installation depth of the impermeable barrier [27] and performing simulation test via hydrological modelling [17].
On the other hand, SWRT has not been yet widely applied, as a consequence of the current lack of dedicated specific machinery for installation. No scientific papers dealing with evaluation or description of a machine for SWRT installation has been indeed found in literature. This represents a crucial point for the development of such technology, considering that it is not possible to put into practice an innovative solution in agriculture sector without solving the mechanical topic [34].
According to what written above this study represents the first evaluation of the working performance of a machinery for the installation of SWRT.
2. MATERIALS AND METHODS
2.1 Description of the SWRT prototype
The prototype was developed within the activities of project MEDIOPUNTIA. It consists of a machine which is able to open a ditch, to install an impermeable film and to reclose the ditch.
The base for the prototype, built by Officine Gottardo sas, in Ormelle (TV), consists of a double-wheel central excavation ditcher. Such kind of machine is usually used for excavating ditches in agricultural fields. The digging capacity of the machinery is given by the rotary wheels, which are laterally mounted to that main framework (Figure 1). They are powered using the Tractor’s PTO set at 1000 rpm, the power required is approximately 100 kW and weight of the machinery is estimated in 2000 kg. The depth of the ditch can be adjusted using the 3-point hitch of the tractor: the lower the hitch, the bigger the reservoir.
Figure 1. Double-wheel central excavation ditcher.
The shape of the ditch is reported in Figure 2.
Figure 2. V-shaped profile of the soil excavated by the rotors.
The excavated soil is collected and returned to ground just above the impermeable film, in order to allow the cultivation. The soil is directly collected from the wheels and conveyed to the rear part of the machine where the impermeable film is already laid down on the ditch. To allow for this task, a couple of conveyers (one per wheel) were installed above the machinery (Figure 3). Positioned behind the wheels, two arms will carry the impermeable roll which is unrolled accordingly with the onward movement of the machinery on the field.
Figure 3. Modifications applied to the machinery to drive the soil above the impermeable film. The film is carried by two arms in rear position.
To allow for a reduced time of the roll installation on its support, an electric winch was positioned in the central part of the machine (Figure 4).
Figure 4. Vision of the SWRT prototype. In the middle of the two conveyers it is possible to notice the electric winch installed to accelerate the positioning of the impermeable film roll.
Another applied modification regards the installation of two mobile screens at the end of each conveyer (Figure 5) in order to have the possibility of direct the spread of soil in exit from the conveyers. Thus to create a ridge downhill in order to collect also the run off water.
Figura 5. Visione del retro del prototipo SWT.
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ACKNOWLEDGEMENTS
The work was performed in the framework of the ERANETMED3-204 project MEDIOPUNTIA (“Introducing cactus plantations (Opuntia spp.) and smart water management systems in marginal lands of Egypt and Morocco to drive rural renaissance in the Mediterranean Region”).
