Abstract
Land degradation due to poor land management is the main constraint for
agricultural productivity globally. Many developing countries have
developed national soil and water conservation (SWC) program however,
sustainable land management is not yet attained. To understand the
effect of SWC measures, a bibliographical review was carried out from
the Scopus Elsevier citation database from 1990 to 2017 using the
keywords “Effect of SWC” in the title, abstract or keywords. Total
amounts of 869 documents were found, and synthesis was made from these
scientific journal articles to assess the main biophysical dynamics of
watersheds due to SWC measures and to highlight the research gaps. The
synthesis revealed that SWC measures in agricultural landscapes have a
significant impact on water and sediment connectivity and therefore on
the erosion process. It potentially slows the runoff generation, reduces
the soil particle detachment and sediment transport; and subsequently,
enhances soil moisture and nutrient availability. However, many research
article outputs and recommendations focus on plot and small-scale
watersheds and are based on short-term or intermediate time frame
results. Since there is a lack of sufficient recommendations and
scientific evidence founded on long-term observations strong
scientifically based feedback on the dynamics of soil properties,
surface runoff and erosion processes at watershed and parcel scale
should be drawn based on both short- and long-term evidence and
considering connectivity and natural-based solutions. This will help to
develop and promote ecologically sound, economically viable and socially
acceptable conservation measures in agricultural watersheds.
KEYWORDS: biophysical dynamics, short and long-term effect, soil
property, surface runoff, soil erosion, research gaps
Introduction
Land degradation is one of the main constraints for agricultural
productivity (Pender, Nkonya, Jagger, Sserunkuuma, & Ssali, 2004;
Taddese, 2001) in areas that have undulating topography, erratic and
intense rainfall and, unsustainable land management practices (Liu et
al., 2013). The extent of land degradation in terms of agricultural
productivity has become the critical challenge for those countries,
which mainly depends their economy on agriculture (Hurni & Pimentel,
1993). The concern of land degradation is a critical among other global
issues because of its impact on world food security and quality of the
environment (Alam, 2014; Eswaran, Lal, & Reich, 2001).
The global importance of soil conservation and the control and
mitigation of land degradation are more highly recognized now than at
any time in the past. To mitigate land degradation, many developing
countries have developed national SWC programs and it has brought
relative improvement in land productivity. However, sustainable land
management is not yet attained (Desta, Carucci, Wendem, & Abebe, 2005).
This is mainly because of the lack of sufficient scientifically based
evidence and recommendations about the requirements, improvement and
impacts of SWC measures (Hurni, 1999; Nyssen et al., 2010). Since
watersheds characterisitics change structurally and functionally after
intervention, understanding the biophysical dynamics of watersheds are a
very crucial issue for sustainable utilization of agricultural
watersheds (Amsalu, Stroosnijder, & Graaff, 2007; Nyssen et al., 2004).
Therefore, considering the change in biophysical characteristics of
watersheds is of primary importance in strengthen knowledge about the
impact of different SWC measures for further scaling up of SWC
technologies (Negussie, Urbaniak, & Zalewski, 2011).
This review paper explores the effect of SWC measure on watershed
biophysical (soil properties, surface runoff, erosion and soil loss)
dynamics at different spatial and temporal scale for better
understanding of landscape biophysical parameters, which are of great
interest for land managers and policy makers. Therefore information and
knowledge is needed on adequate scale. Spatially on the farm or
watershed scale; temporally annualy or decadal scale. Therefore, this
review aims: i) to assess the available scientific documents and
recommendations in the field of SWC on different spatial and temporal
scales; ii) to identify the research gaps and to propose feature
research objectives; iii) to understand the key biophysical dynamics
(soil properties, surface runoff and soil erosion) of watershed due to
SWC measures at different temporal and spatial scale.
Material and Methods
SWC measures such as soil bunds, terraces, tillage types, cover
management, grass strips, watershed management (before and after
intervention) and other agronomic measures were evaluated on their
effect on catchment erosion, runoff and soil properties dynamics. The
Scopus Elsevier database was used to search for research papers using
the key words “Effect of SWC” on soil properties, runoff and soil loss
in the title, abstract or keywords from 1990 to 2017. The resulting 869
documents (Table 1) comprised of 79% were journal articles, 12 %
conference papers and, the remaining (9 %) are reviewed book chapters,
conference reviews and articles in press. The bibliographical search was
further expanded using the words “effect of SWC on erosion and soil
loss”, “effect of SWC on runoff” and, “effect of SWC on soil
properties” to address the specific biophysical parameters (soil
properties, surface runoff and, erosion and soil loss) of agricultural
watersheds worldwide (Table 2). From the second search, we found 158
documents with the word ‘erosion and soil loss’, 96 documents with the
word ‘runoff’ and 138 documents with the word ‘soil properties’. After
this review, a critical selection synthesis based on the watershed
approach was carried out to evaluate the effect of SWC on the
biophysical parameters found in the different scientific journal
articles, conference papers and book chapters. This have been summarized
into three main watershed characteristics, (i) soil properties (physical
and chemical), (ii) surface runoff, and (iii) erosion and soil loss..
Data on the effect of SWC on soil properties, run-off, erosion and soil
loss were organized in a database using Microsoft Excel. Once the data
were organized and structured, different descriptive statistics were
conducted to synthesize the data.
Results
Physical, biological and agronomic SWC measures are a major component of
agricultural watershed management, which affects surface roughness and
cover of the soil, whereby it potentially has a positive impact on soil
retention against raindrops and running water and soil chemical,
physical and biological improvement. Table 3 provides a summary of
previous studies describing the effect of SWC measures on soil physical
and chemical properties. The evaluation was addressed the major soil
properties such as organic matter (OM), organic carbon (OC), bulk
density, soil moisture content, total nitrogen (TN), available
Phosphorus (Ava.P) and, pH. As shown in table 3, the effect of SWC
practices on soil properties could be positive (+), negative (−) or
neutral (0). Regarding soil properties, from the total reviewed papers,
71 % of the research articles were carried out at plot level while the
remaining 29% was at catchment scale. The synthesis revealed that from
the total observation 75% showed that physical, biological and
agronomic SWC measures has a positive effect on most soil properties
while the remaining 25 % indicate SWC measures has negative or neutral
effects on the selected soil properties. In all cases it is observed
that, physical, biological and integrated SWC measures such as soil
bunds, stone bunds, mulching, and agronomic measures has an impact on
the improvement in soil properties as compared to the untreated
conditions. It is also observed that individual SWC measures has less
effect on the improvement of soil physical and chemical properties as
compare to the integrated SWC measures.
The results in Table 4 present the condition of surface runoff with and
without of SWC measures. The review indicated that, most of the research
reports (70%) carried out on the bases of plot level observations
whereas catchment level researches accounts only 30 %. As indicated in
the table below, the synthesis shows that, SWC measures, such as
physical structures, biological measures and agronomic measures have a
significant impact on the improvement of hydrological process through
infiltration enhancement and surface runoff reduction. From the total
reviewed papers, 87% articles indicated that, surface runoff has
reduced significantly due to SWC measures as compared to the untreated
once; while 13% articles that shows there is an increment of runoff by
9% as a result of graded SWC measures. The reduction in surface runoff
ranged from 24% to more than 100%. The increment in runoff due to
graded SWC structures are mainly because of its structural designed to
expose direct runoff safely from the farm without scoring effect. The
review showed that physical SWC structures as terrace reduces the
surface runoff by three times.
As presented in Table 5, the review also addressed the effect of
different SWC measures on erosion and soil loss. The synthesis showed
that among the reviewed papers 53 % of research articles were carried
out based on plot level experiments and the remaining 47 % belongs to
catchment scale researches, which has been discussed in section 4.3. All
the reviewed articles indicated that SWC measure has a positive effect
on soil loss reduction at plot and catchment scale. The reduction in of
soil loss ranges from 15% to more than 100% as compared to untreated
(control). The review showed that in some cases physical SWC structures
such as terrace reduces soil loss by three times, than the untreated
plots.
- Discussion
- Effect of SWC on soil properties
Soil properties are one of the major governing factors in erosion
processes in the landscape particularly for detachment phase (Wei et
al., 2012) which mainly is affected by soil and water management
interventions(Abegaz et al.,, 2016; Su et al., 2010). The dynamics of
soil properties are always depending on both land management practices
and the inherent properties of the soil (Jenny, 1941; RE, 2013).
Depending on how it is managed, soil is a sink of plant nutrients like
carbon and nitrogen, microorganisms, water, and other toxic substrates
(Lal & Pierce, 1991) and it determines what an ecosystem will look like
in terms of production (Hati et al., 2013). SWC measures are the major
component of soil and water management practices in the catchment, which
potentially affects soil chemical, physical and biological improvement
(Castellini & Ventrella, 2012; Green & Erskine, 2011). In many cases,
agronomic and vegetative SWC measures such as mulching, compost,
conservation tillage, residue management and other tillage practices
have a significant contribution on the improvement of soil physical and
chemical properties (Mekuria et al., 2007; Zhang et al., 2009) when it
implements integrated with physical SWC measures (Bienes et al.,2016;
Hailu, 2017). As shown in table 3, physical SWC measures such as stone
terraces, soil bunds, Fnya juu and contour ridges mainly governs the
movement of soil nutrients in the landscape through sediment trapping
(Guzman & Al-Kaisi, 2011; Hailu, 2017; Mekonnen et al.,, 2015; Xu et
al., 2012). According to G/mariam (2015), the treated cultivate land
with integrated SWC measures had significantly higher mean percentage of
SOM and TN compared to untreated cultivated lands. The finding in the
Chinese catchment indicated that, from the total transported soil and
nutrients 94.8 % had been locally retained in the catchment due to SWC
measures (Xu et al., 2012). Similarly, Demelash & Stahr (2010) and
Hailu (2017) also reported that farmland with physical SWC measures had
higher total nitrogen and organic matter and lower bulk density compared
toland without SWC measures.
Al-Seekh and Mohammad (2009) and Fu et al. (2003) reported that
conservation practices were found to be more effective in increasing
soil moisture storage in dryland areas. The research conducted by
Zougmoré et al. (2004) indicated that, the stone rows and grass strips
combined with compost reduced runoff and increased soil water storage.
These combined practices could create sound soil water conditions and
were able to satisfy the crop’s water demand (Mengistu et al., 2016). In
general, this review and synthesis showed that the implementation of
physical, biological and integrated soil and water conservation
practices such as soil bunds, stone bunds, mulching, and agronomic
measures reduces soil erosion by water and thereby decreases fertile
soil loss and enhance soil physical and chemical properties. However,
when we consider the dynamics of soil properties, it is better to have
long term time series to have have a good chance to have captured all
temporal dynamics of soil fertility and land productivity.
Surface Runoff
Surface runoff might occur when rainfall or run-on form adjacent areas
exceeds the infiltration rate of the soil. Since surface runoff is the
primary agent for soil erosion, studying the extent and its process is
important to understand the hydrologic impact of SWC measures in a
watershed. Runoff is influenced by catchment factors such as topography
(Cerdà & García-Fayos, 1997; Nyssen et al., 2008), vegetation (Cerdà &
Doerr, 2005; Keesstra, 2007), infiltration rates and soil storage
capacity (Bouchnak et al., 2009; Desta et al., 2005). SWC measures
affects surface roughness and cover of the soil, whereby it potentially
has an impact on hydrological process particularly surface runoff
(Castellini & Ventrella, 2012; Green & Erskine, 2011). Al-Seekh &
Mohammad (2009) also indicated tha the amount of runoff varied under
different management conditions and it is affected by soil surface
roughness and vegetation cover. Similarly, a study carried out on four
differentially treated catchments in the northwest part of Punjab in
India by Kukal & Bawa (2013) who stated that the runoff was 71% higher
in untreated catchment than in treated catchments. This is because SWC
measures divide the slopes in a landscape and reducing with this the
quantity and speed of surface runoff by increasing the time of
concentration.
Similarly, Mengistu et al. (2016) and Amare et al. (2014) reported that,
surface runoff generated from soil bund and Fanya juu was significantly
lower than in the control plots. The study conducted in West Bank showed
that, runoff reduced by 65–85% in stone terraces and semicircle bunds
compared to the control at the semi-humid site (Al-Seekh & Mohammad,
2009). SWC practices also have an impact on catchment landscape terrace
development and this plays a crucial role in reducing water erosion and
surface runoff (Carla McCullough et al.,, 2008; Nyssen et al., 2010).
This is due to the fact that terrace development could decrease the
parcel slope angle and slope length, which finally leads to minimizing
the runoff extent and speed. Wang et al. (2011) indicated that, from a
natural rainfall experiment plot with reverse-slope terraces could cut
down runoff by 65.3%. Surface runoff amount and runoff coefficient of
control slope fields increased by 21.5 - 41.0 % and 27.5 - 69.7 %
respectively as compared to those of sloping terraces (Zhang et al.,,
2008).
Vegetative and agronomic SWC measures plays an important role in runoff
reduction due to the enhancement of infiltration rate and minimize
runoff velocity (Adimassu et al., , 2012; Mitiku et al., 2006).
According to Liu et al., (2017), grass hedges decreased runoff by
50.1%–60.7% compared with the control plots without grass hedges.
This is because grass strips decrease the flow velocity and filter
sediment from the flow (Cao et al., 2015). A similar study, which was
conducted in Spain, indicated that, the application of straw mulching at
different mulching rates significantly delayed the runoff time compared
with uncovered treatments (Keesstra et al., 2016; Zhang et al., 2015).
Zhang et al. (2015) also reported that compared with no straw mulching
treatment, 30% straw mulching treatment significantly reduced total
runoff by 17.9%-38.7%.
Soil Erosion and Sediment Loss
SWC measures in the watershed management is a key factor influencing
soil erosion and hydrological processes and sediment connectivity on
agricultural land. These management practices, such as physical SWC
structures, vegetative and mulch covers have a significant effect on
preventing raindrops from hitting the soil and causing detachment and
transportation of the soil particles along the slope. Landscape
topographical variations due to SWC measures change local flow
velocities, and the process of soil delivery and sedimentation, thereby
affecting the intensity of soil erosion and water and sediment
connectivity (Keesstra et al., 2009; Liu et al., 2013). The impact of
SWC in the watershed management could be explained in terms of
minimizing soil erosion rates and increase soil moisture availability by
diverting the channels and water movement and thereby reduce
sedimentation (Liu et al., 2013). Mekonnen et al. (2017) also reported
that, SWC barriers have a significant role to trap sediment within the
catchment by decreasing soil erosion and enhancing the rate of
sedimentation within a catchment through channel dis-connectivity.
Setegn et al., (2010a) reported that, a single slope discretization,
gave a lower average monthly sediment yield than other scenarios in
comparison with the measured sediment yield at the outlet of the
watershed.
Physical SWC measures, like terracing, bunds and other barriers are also
the main components in soil loss reduction. The study in China under
different simulated rainfall indicated that sediment yields decreased by
terrace construction by more than 20% (Liu et al., 2013). Similarly, in
India, the temporal variation of soil loss on differently treated
catchments was tested. The result indicated that, soil loss during the
initial years after SWC measure implementation in the catchment, the
fully terraced catchment had the lowest soil loss (25.2 Mg
ha-1 y-1) and the highest (43.3 Mg
ha-1 y-1) was recorded in the
untreated catchment. However, during the later period the trends
reversed (Kukal & Bawa, 2013). This could be simply due to the fact
that during initial stage SWC structures retained the sediments on the
upstream side of the structure whereas in the final stage, the
deposition of sediments resulted in complete siltation may increases
soil erosion (Fiener & Auerswald, 2006; Kukal & Bawa, 2013).
Therefore, understanding on the effect of different SWC measures on
biophysical dynamics of watersheds at different temporal scale is very
important for sustainable land management.
The study that was conducted in West Bank showed that, sediment is
reduced by 58–69% in stone terraces and semi-circle bunds compared to
the control plot which doesn’t receive any SWC structures (Al-Seekh &
Mohammad, 2009). Zhang et al. (2008) reported that the sediment yields
on the slope fields without any SWC measures were higher (34.41 - 331.67
%) than those on sloping terraces; due to the fact that terracing plays
a significant role on soil erosion reduction in cultivated sloppy areas.
Like previous studies, Shen et al., (2010) and (Li et al., 2011) found
that planting vegetation on dike terrace was beneficial in controlling
soil erosion compared to farming of sloped land. Plant roots have been
found to hold soil in place and prevent soil from collapsing during
rainstorm events (Baets et al., 2006; Dong et al., 2015). Therefore, it
is recommended that optimum coverage of sloppy areas is very important
to stabilize and protect the bottom of sloped land against erosion (Liu
et al., 2013).
Main research gaps