Topsoiling and Subsoiling as Rainwater Harvesting Techniques for Arid Climates: a Case Study from Sudan1

SHAMSEDDIN, Musaa and ALI, Adeebb

a Water Management and Irrigation Institute, University of Gezira, Wadmedani, Sudan, email: shams_id@yahoo.com b Water Management and Irrigation Institute, University of Gezira, Wadmedani, Sudan, email: amadeeb@yahoo.com

Abstract — Yield of rainfed crops has recently showed a declining trend due to rainwater mismanagement aspects rather than water availability per se in Sudan. The objective of this study was to evaluate and recommend the best in situ rainwater harvesting techniques that can ensure sustainable crop productions under arid conditions of the central Sudan. This was done through monitoring of soil moisture content, soil moisture depth, infiltration rate, crop evapotranspiration (ETc), yield and green water footprint. Data collection lasted for two consecutive experimental seasons, using the 1-factor completely randomized design. The tested in situ rainwater harvesting techniques were topsoiling and subsoiling tillage. The conventional bund rainwater harvesting techniques (locally known as Terraces) coupled with zero tillage that is adopted widely by the local farmers was taken as a control. It was found that the collected data of both topsoiling and subsoiling superior were superior over the control, suggesting that topsoiling and subsoiling were win-win techniques (increasing crop yield and improving soil hydrological properties) under the arid conditions of the central Sudan. This was questioned the current Sudanese governmental belief on the zero tillage for upgrading the rainfed agriculture.
Keywords — topsoiling; subsoiling; zero tillage; rainwater harvesting; arid conditions

1 Introduction

The dramatic growth of human population necessitates the production of more food, which would put more pressures on the limited fresh water resources. The hard approach of construction of dams had unanticipated social, economic and environmental costs, which should be well considered (Gleick, 2003: 1524). Thus, in arid climates water policy shall focus less on engineering problems and more on the efficient use of existing water resources (Bijan and Ali, 2002: 128).

Arid and semi arid lands cover around 30% of the total world area (Sivakumar et al., 2005: 31), which was projected to increase by 5-8% due to climate changes (IPCC 2007: 50). The low and irregular precipitation of these environments resulted in difficulty coping with environmental stress, droughts and climatic change (Sivakumar et al., 2005: 52), leading to massive food insecurity, environmental degradation, poverty and incapability to combat climatic changes (IPCC, 2007: 65; Vohland and Barry, 2009: 120).

Livelihood security in Eastern and Southern Africa is strongly dependent on rainfall distribution and land management practices among small holders' farmers as the rainfed agricultural sector produces over 95% of the food (Rockström, 2000: 1). However, water rather than land is the limiting factor. Thus, future food production has had to be achieved through a better field water management. This calls for water conservation practices or a green revolution, especially in rainfed agriculture (Rockström et al., 2009: 1). Among these practices rainwater harvesting is tremendously prompted by the national and international water related agencies (Vohland and Barry, 2009: 120). Positive impacts of rainwater harvesting techniques (RWHT) on increasing yield of rainfed crops, especially in arid and semi arid areas, have been thoroughly investigated and shown successful results (Tian et al., 2003; Motsi et al., 2004; Li et al., 2006; Ito et al., 2007; Rockström et al., 2010). Moreover, RWHT can stabilize agricultural productions and make them more productive and more resilient toward climatic changes (Vohland and Barry, 2009: 120).

The ensuring continuation into the future for optimal generation of livelihoods will depend on the sustainability of RWHT, which in turn depends on reliable water supply and production, increase rainwater productivity and minimal negative impacts on natural resources (Pachpute et al., 2009: 2817). Vohland and Barry (2009: 120) examined the effect of RWHT on the sustainability and found that RWHT improved infiltration and groundwater recharge, enriched soil nutrients, increased biomass and yield and improved farmers’ income. However, the low rate of adoption of RWHT was the main problem, since RWHT are practiced currently only on 3% out of 960 Mha of arable and permanent crops in Africa, Asia and Latin America (Pretty et al., 2003: 217). The inadequate policy and budgetary allocation, lack of technical know-how and limited scientific and socioeconomic knowledge on rainwater harvesting are the main factors that limit the success of RWHT for upgrading smallholder livelihoods in Sub Saharn Africa (Pachpute et al., 2009: 2817).

The Nile Water Agreements in 1959 has stipulated a limited share for Sudan in the Nile River. This has constrained the irrigation development in Sudan. Accordingly, the growth rate of the agricultural sector is dominated by the performance of rainfed sector (Ayoub, 1999: 489). Two types of rainfed agriculture are practiced in Sudan: the dominant traditional and the mechanized sectors. Crop yields under rainfed agriculture are far beyond its potential, resulting in a poor sustainability. A report by the Food and Agriculture Organization of the United Nations, FAO (2006: 21) stated that farmers have had to increase their cultivated area by 75% in order to maintain the same level of the 1970/1971 income. This is attributed to rainfall variability and loss of soil fertility (Ayoub, 1999: 489). Shamseddin (2009: 153) attributed the low yield to water mismanagement. In the period 1970s-1990s, a number of rainwater harvesting projects were implemented in order to combat the effects of drought. However few succeeded due to the lack of technical know-how, and to inappropriate approaches of selection with regards to the prevailing socio-economic conditions (FAO, 2011a).

Recently, the Sudanese government has believed that zero tillage practices have the potential for improving rainfed agriculture performance (FAO, 2011). However, a mixture of successful and unsuccessful results of adopting zero-tillage was documented worldwide (Unger et al., 1998; Mupangwa et al., 2006; Ito et al., 2007; Oloro et al., 2007; Wang et al., 2007). Mati (2004: 14) called that in East Africa zero tillage was not promising due to poor infiltration (as soils are easily self-sealing) and costs of herbicides being prohibitive.

The construction of surrounding dykes on a flat land with no till and plant distances of a one meter (20000 plant ha) is the locally widely adopted practice by poor traditional rainfed farmers, in the central Sudan. On one hand, this practice can be considered as a rainwater harvesting technique, as it concentrates within-field sheet runoffs to the crop. On the other hand, it is a zero tillage practice. Evaluating this conventional practice (hereafter referred to as BT) against furrows (FT) and chisel (CT) practices was the general aim of this study.

2  Materials and Methods

The study area is located in the central Sudan. The climate is arid and characterized by a low rainfall concentrated to three months (July-September). Seasonal rainfall ranges from 200-300 mm, coupled with a high evapotranspiration rate of 2600 mm, with a daily average ranges from 5.5 – 9.5 mm. The length of growing period is 75-90 days. This gives a cumulative potential evapotranspiration of 460-560 mm during the growing period. Thus, water supply is very limited.

Rainfall and length of dry period are the most important climatic variables, in the region (FAO 2006). The region is susceptible to repeated drought cycles, especially during the periods 1970s and 1980s that have witnessed the severe drought, resulting in catastrophic consequences, i.e. mass lost of souls, destroyed vegetation covers, failure in biomass production, displacement and social stability (Osman and Shamseldin, 2002; 1862). Thus, the drought has been related to famine, civil unrest, ill-health and desertification and in turns as a constraint upon development in Sudan (Hulme, 1987: 326; Osman and Shamseldin, 2002: 1862). Hulme (1986: 44) stated serious deterioration of growing conditions in the arid climate (200-300 mm) of the central Sudan. Thus, drought and dry spells (short periods of water stress due to the poor distribution of rainfall) often caused serious impacts on crop yield, especially if dry spells occur during a critical water sensitive development stage, e.g. flowering stage (Rockström, 2000: 3).

2.1 The field experiments

The experiments last for two consecutive seasons (2006/2007-2007/2008), at the Faculty of Agricultural Sciences, University of Gezira (14.5º N and 33.5º E). Since the primary goal of this study was to draw a conclusion on impacts of tillage on the rainfed crop production, the 1-factor completely randomize design was applied. The tillage factor was separated into two levels, viz. topsoiling, i.e. furrow, and subsoiling, i.e. chisel), that are in-situ RWHT (defined as maximizing benefits from rainfall where it falls). The control was the locally widespread bund (locally known as Terraces) RWHT coupled with zero tillage. Thus, the experiment total number of runs was six; each run has a size of 13 x 70 m. Selected phenological data, i.e. root length development, leaf number, disease and insect situation were observed, too.

The experimental site belongs to the central clay plain, where the soil is Vertisols with a clay percent of 52-58% (Elias et al., 2001: 153). The soil was prepared in early July. The cultivated crop was sorghum (the dominant staple food crop in the country). In order to avoid water stagnation problems, plants holes were placed a little bit higher than the beds of the furrows (0.7 m wide); using a conventional planting method, traditionally known as Saluka,, with 0.2 - 0.3 m distance per hole (3 - 4 seeds per hole). The same procedure was followed on chisel plots, but on flat lands. On the control plots the distance between holes was 0.7 – 0.8 m and 3-4 seeds per hole. The re-planting process was done after one week of germination. Thinning and weeds clearance operations were carried out timely by direct labor. Soil moisture contents were measured using the gravimetric sampling method. The crop evapotranspiration (ETc, mm/day) was estimated following the procedure mentioned in (Mekonnen and Hoekstra, 2010: 9) as follows:

if

otherwise

Where, Ks is the dimensionless water stress coefficient, depending on the available soil moisture in the root zone), Kc is the crop coefficient and ETo is the reference evapotranspiration (mm day-1), S[t] is the actual available soil moisture at time t, Smax is the maximum available soil moisture in the root zone (mm), and p is the dimensionless fraction of Smax that crop can extract without suffering a water stress. Values of p were taken as 0.6-0.8. The gravimetric sampling accuracy was found as 19.9%. Kc values were taken from Adam (2005). ETo was calculated using CROPWAT 8.0 program on the basis of Penmann-Monteith, where the necessary climatic data were obtained from Wadmedani station. Tests of significance were done using t-test. In situ infiltration tests were carried out using a double ring infiltrometer, based on the empirical equation of Kostiakov-Lewis:

where Z stands for the cumulative infiltration (L), t is the opportunity time, c is the basic infiltration rate (L T-1), k is a coefficient representing the water layer that enters the soil during the first unit of time (L T-1) and a is a coefficient that usually ranges between 0.2-0.8. The efficient use of water was indicated through the water footprint concept (WFP, m3/kg), following the procedure mentioned in Hoekstra et al. (2009: 26) where grain yield and green crop evapotranspiration data were used [WFP ≈ ETc/yield]. Tests of significance were done using t-test in the SPSS statistical package.

3  Results and Discussions

3.1  Hydro-climatic conditions

The average rainfall of the region was found to be 268 mm (1975-2005), of which 97% was concentrated to two months (July and August). Therefore the total rainfalls of the first and the second experimental seasons, 270 mm and 327 mm respectively could represent normal and above normal hydrological conditions, respectively. However, the first season experienced a better rainfall temporal distribution as during the second season two dry spells of 17 days for each were recorded. Also, during the second season the shorter time spans between rainfall events (Fig.1) resulted in water stagnation problems. Therefore, the first season was better than the second from a rainfed hydrological conditions point of view.

According to Figure (1) the proper sowing date could be very important in combating drought and climate change. During the second season the majority of rainfall events (62%) were received prior to the sowing date, compared to only 28% of the first season (most of the local farmers sow after 20th of July). Therefore, it is misleading to judge the agricultural season as good or bad without considering the sowing dates. Therefore, there is a badly need to carry out in depth studies on the impacts of climate change on the proper sowing date.

Figure 1: Rainfall distributions during the first (a) and the second (b) experimental seasons in the central Sudan. DAS refers to days after sowing

3.2  The implementation of in situ RWHTs

During the normal hydrological conditions of the first season, the implementation of FT and CT resulted in significant increasing in the soil moisture content (P ≈ 0.01), compared to the BT. This is confirmed the results obtained by Motsi et al. (2004: 1071) who found that tied ridges retained the highest soil moisture content compared to the flat plowing in Zimbabwe. Also, the average maximum wetted profiles were found deeper at FT and CT plots (1.0 m depth) compared to 0.6 m of the BT. Fig. (2a) showed that during the initial and vegetative growth stages (15 July-10 September) the soil moisture content of the BT was larger than those of FT and CT, but the opposite holds true for the remaining growth stages. This was attributed to three reasons: (1) the large distances between plants of the BT (1.0 m a part) lead to large bare soils, which in turn increased the evaporation part; (2) the FT and CT practices were capable to retain more soil moisture from very low rainfall event conditions that overcome the terminal droughts. Vadez et al. (2011: 652) found that the transpiration efficiency could significantly explain yield differences under terminal droughts in sorghum; (3) the redistribution of soil moisture was better under FT and CT. Laddha and Totawat (1997: 241) found that deep tillage was superior to shallow tillage practices as it significantly reduces the bulk density, increases soil porosity and profile water contents, compared to zero tillage, in a sandy loam soil condition. According to the results farmers should consider two essential technical points under arid conditions: the first is that replanting is unavoidable process under arid rainfed condition since in this study replanting process compensated approximately 30% of the recommended plants intensity, i.e. most of the rainfed-farmers do ignore the replanting process. The second point is the placing of planting holes a little bit higher than the beds of the furrows so as to avoid the water-logging conditions that usually follow the consecutive rain storms events in clay soils. Considering these two points it could be safely saying that topsoiling and subsoiling were better than the zero tillage in conserving soil moisture during the normal hydrological conditions of the arid environments.

Figure 2: Soil moisture content during the first (a) and second (b) seasons

The significant differences found in soil moisture contents of the first season lead to significant variation (P≈ 0.01) in daily ETc values of the BT, FT and CT (Fig. 3a). However, the seasonal ETc values showed insignificant differences (Fig. 4). Thus, rather than the seasonal, the daily hydrological differences would have the significant contribution on the progress of the growth season in the arid environments. It is therefore the daily time span data are more valuable than the monthly and yearly data in order to evaluate or predict the performance of the arid rainfed agriculture.

Table (1) shows the final obtained sorghum grain yields, which were found significantly different (P ≈ 0.01). It is obvious that both of FT and CT produced the highest yields compared to the BT. Of the tested systems the FT is the best, suggesting that during a drier season furrows system is the best practice to adopt under arid climates. Noting that during the first season the majority of rainfall events (65%) was less than the threshold for initiating sheet flows that starting at 13 mm (Shamseddin, 2009: 88). For clarification, 65% of the received rainfall events were found less than 10 mm, 20% were between 11- 20 mm, 5% were between 21-30 mm and 10% were more than 40 mm. It is therefore furrows system is capable to capture more rainwater under low rainfall amounts conditions of the central Sudan.

The wet conditions of the second season lead to insignificant differences in soil moisture contents (Fig. 2b), resulting in insignificant differences among the daily and seasonal ETc values (Fig. 3b). The final yields however of both CT and FT were found significantly higher (P ≈ 0.01) than that of the BT, as shown in Table (1). Those differences were attributed to the impacts of both dry spells and water stagnation problems as previously discussed. Accordingly, (FT) and subsoiling (CT) practices are more successful than the zero tillage (BT) in tolerating dry spells and water stagnation problems that can be likely occur during wet years under the arid vertisols of the central Sudan. It is worth mentioning that topsoiling is frequently adopted in the central Sudan, however, it was mostly performed against the contour. In addition, the monoculture of sorghum crop jeopardized the sustainability of the rainfed system (Blokhuis, 1993: 45). Thus, more extension effort is needed to raise the awareness of the local farmers.

Figure 3: Actual evapotranspiration (ETc) values for the first (a) and the second season (b)
Figure 4: The seasonal crop evapotranspiration during the first (season1) and the second (season2) experimental seasons for each cropping system
Table 1: Sorghum yields (kg/ha) for the topsoiling (FT), subsoiling (CT) and the control (BT) during the first (season1) and the second experimental seasons
 FTCTBT
First season18801623493
Second season39662590

3.3  The infiltration tests

It is likely that thunderstorms enhance the formation of the structural soil crust under the arid vertisols, resulting in impermeable layers which cause low infiltration rate and restrict seedling emergence. Hence, tillage practices such as FT and CT could destroy those impermeable layers, facilitating penetration of water and reducing thus evaporation losses. On the other hand, due to soil moisture fluctuations cracks are used to be developed under vertisols, enhancing entering of water into the soil and also could increase substantially evaporation losses (Elias et al., 2001: 155). Thus, the infiltration tests were carefully generated at free cracking conditions, i.e. the average soil moisture content was 46 mm for the upper 30 cm depth (the total water holding capacity is 230 mm/m).

Of the infiltration data, those of CT and FT were found greater than those of BT. The basic infiltration rates were found to be 4.8 cm h−1 for CT, 4.2 cm h−1 for FT compared to 1.8 cm h−1 for BT (data of BT infiltration test is taken from WMII database). Regardless of the insignificant differences between FT and CT data, the cumulative infiltrated volume of the CT showed a large standard deviation of 176.7 mm compared to only 61.3 mm for FT. Thus, the soil hydrological characteristics of the topsoiling and subsoiling were better than the zero tillage. This is attributed to the unmet principle of leaving at least 30% of crops residual (zero tillage) under arid conditions due to the low plant density used, which is used to be taken as a drought tolerance technique by local farmers. Mohanty et al. (2007: 420) found that the basic infiltration rate of vertisols was greater after the subsoiling tillage (5.65 cm h−1) than the topsoiling tillage (1.84 cm h−1), under rainfed conditions. However, Reynolds et al. (1995: 117) found no consistent trends for the near-saturated hydraulic conductivity values (almost equals the basic infiltration rate) on tilled and untilled soils. Generally, Vohland and Barry (2009: 122) indicated that in situ rainwater harvesting practices improve the infiltration and groundwater recharge. However, in this study only tillage practices would result in similar results.

3.4  The field water footprint

The impacts of in situ RWHT on the crop water use were investigated through the concept "water footprint, WFP". The normal conditions of the first season resulted in WFP values of 1100, 1100 and 4000 m3/ ton for FT, CT and BT, respectively. During the second season, these WFP values have increased by 90%, 16% and 110% because se of dry spells and water stagnation problems (Fig. 5). Obviously the zero tillage practice (BT) tends to reduce crop yield during wet conditions, agreeing with the results of Wang et al. (2007: 239) who found a yield reduction of 10-15% with no-till practice; and disagreeing with their results that zero tillage performs better in dry years, compared to conventional tillage practices in the arid lands of China.

Globally, Mekonnen and Hoekstra (2010: 17) estimated the green WFP of the rainfed sorghum at 2857 m3/ ton, suggesting that the WFP under the arid climate of the central Sudan could not approach the global one without the adoption of rainwater harvesting techniques. Succinctly, FT and CT could provide rooms for water savings (85%) in the arid conditions of the central Sudan.

Figure 5: Water footprint (WFP) for each cropping systems during the first (season1) and the second (season2) experimental seasons in the central Sudan

4  Conclusion

Due to the limited irrigation water supply, rainfed agriculture is the largest contributor for livelihood generation in the central Sudan. Thus, it has large effects in the sustainability of natural resources. Therefore, practices leading to a sustainable rainfed agricultural system are very important. Currently, most of rainfed farmers adopt widely the bunds rainwater harvesting technique, which was found unsustainable practice relative to the topsoiling and subsoiling, which could provide win-win benefits, i.e. from agricultural (increase crop yield) and soil management (improve soil properties) point of views. The adoption of the topsoiling and subsoiling practices among rainfed farmers however requires political will, facilitation of proper equipments and capacity building programs among agricultural extensionists and farmers as well. The successful story of zero tillage is a site specific. Thus, there is a badly need for validating the Sudanese government believes on zero tillage before expanding its adoption.

Acknowledgement

Part of this study is a Ph. D. study funded by the DAAD. Thanks also extend to ITT, Cologne University of Applied Science, Germany for hosting the first author. Thanks also extend to Prof. Hussein S. Adam, Prof. Seifeddin Hamad and Prof. Adam Ibrahim

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Citation

Shamseddin, M.; Ali, A. (2014): Topsoiling and Subsoiling as Rainwater Harvesting Techniques for Arid Climates: a Case Study from Sudan. In: Planet@Risk, 2(1), Special Issue on Desertification: 40-46, Davos: Global Risk Forum GRF Davos.


1
This article is based on a presentation given during the UNCCD 2nd Scientific Conference on "Economic assessment of desertification, sustainable land management and resilience of arid, semi-arid and dry sub-humid areas", held 9-12 April 2013 in Bonn, Germany (http://2sc.unccd.int/home/).

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