D.D. Poudel et al. Agriculture, Ecosystems and Environment 79 2000 113–127 115
Soil Erosion-Soil Productivity Research Planning Committee, 1981. While the well-tested empirical
Universal Soil Loss Equation USLE can be modi- fied to cope with steepland situations in the tropics
Lo, 1994, process-based models are generally pre- ferred Paningbatan, 1994; Hashim et al., 1995. The
Erosion-Productivity Impact Calculator EPIC model Williams et al., 1984 is a comprehensive field-scale
model that operates on a daily time step King et al., 1996. It is process-based and has been tested exten-
sively in a number of local, regional, and national studies in continental USA and Hawaii Steiner et al.,
1987; Williams, 1991; Phillips et al., 1993; Richard- son and King, 1995; King et al., 1996; Cavero et al.,
1998. EPIC has the capability to simulate complex crop rotations for decades and centuries; it is designed
to help the decision makers to evaluate alternative cropping systems, and predict their socio-economic
and environmental sustainability Cabelguenne et al., 1990; Jones et al., 1991.
Farmer participatory research FPR has been proposed as an approach in developing appropriate
agricultural systems that are indisputably acceptable to farmers yet contribute to the improvement and mainte-
nance of agricultural sustainability and environmental quality Fujisaka, 1989; NRC, 1991; Edwards et al.,
1993; Cox et al., 1996; Rhoades, 1997. Rhoades and Booth 1982 developed the Farmer-Back-to-Farmer
model which was a forerunner to the participatory ap- proach. This model begins and ends with farmer, and it
involves four activities: 1 farmer-scientist diagnosis 2 interdisciplinary team research 3 on-farm testing
and adaptation, and 4 farmer evaluationadaptation. Thus, the farmer is considered as an ‘expert’ member
of the interdisciplinary team and is integrally engaged in problem identification, definition, and solution
design.
In the upper slopes of the Manupali watershed in northern Mindanao in the Philippines, soil erosion on
commercial vegetable farms was reportedly largely responsible for a declining crop productivity. As part
of a larger project on Sustainable Agriculture and Natural Resources Management SANREM-CRSP
NRC, 1991, farmer participatory research on soil erosion management was started in 1994. The ob-
jectives of this research were: 1 to measure soil erosion losses on farmers’ fields using Farmer Partic-
ipatory Research approach, 2 to assess the effects of cropping sequence on soil erosion in steepland veg-
etable systems, and 3 to provide recommendations for choice of cropping sequences to farmers, credit
agencies and agricultural technicians for enhanced production system sustainability.
2. Methods
2.1. Description of the study area The study area is located in the Manupali wa-
tershed 124
◦
47
′
to 125
◦
08
′
E and 7
◦
57
′
to 8
◦
08
′
N Kanemasu et al., 1997 in northern Mindanao, the
Philippines. The soil parent materials were thick de- posits of siliceous volcanic ejecta, either deposited
in place volcanic cone or transported from ups- lope as colluvial or alluvial materials. Elevations
in the watershed range from 320 m above sea level masl to 2938 masl. This watershed has four broad
geomorphic units: the Mountains 1400–1900 masl, the Upper Footslopes 700–1400 masl, the Lower
Footslopes 370–700 masl, and the Alluvial Terraces 320–370 masl West et al., 1997. According to FAO
classification system, soils in the Mountains were Silic Andosols, Haplic Acrisols, and Dystric Cambisols,
while those in the Upper and the Lower Footslopes were Lixic Ferralsols and Haplic Ferralsols as were
those in the Alluvial Terraces.
Mean annual precipitation 1994–1996 measured at a weather station in the watershed 1500 masl was
2825 mm Table 1. Rainfall is not equally distributed throughout the year, but there is normally no month
with 100 mm of rainfall. Mean monthly minimum and maximum air temperatures were 15.5 and 26.4
◦
C, respectively. Major crops grown are corn, sugarcane
Saccharum officinarum L. and rice Oryza sativa L. at the lower elevations, whereas tomato, potato, sweet
pea Pisum sativum L., cauliflower Brassica oler- aceavar. botrytis L., cabbage and other leafy vegeta-
bles are dominant field crops in the upper elevations.
2.2. Formal survey and farmer-scientist diagnosis As shown in Fig. 1, the research study started
with a participatory householdfarm survey conducted in May–June 1994. This survey was done to un-
derstand the existing vegetable production systems,
116 D.D. Poudel et al. Agriculture, Ecosystems and Environment 79 2000 113–127
Table 1 Mean monthly rainfall, temperature, and relative humidity for the research site in the Manupali watershed, Mindanao, the Philippines
1994–1996
a
JAN FEB
MAR APR
MAY JUN
JUL AUG
SEP OCT
NOV DEC
Annual Precipitation mm
150 124
97 164
363 327
194 412
242 421
118 213
2825 Number of days with precipitation
21 16
17 16
24 27
22 23
21 27
20 21
256 Maximum air temperature
◦
C 23.2
23.1 23.8
24.5 24.5
24.1 23.6
23.7 23.4
23.4 26.4
25.4 24.1
Minimum air temperature
◦
C 15.5
15.2 15.5
16 16.8
16.7 16.6
16.4 16.7
16.5 17.4
17.5 16.4
Relative humidity 88
87 85
85 86
89 86
87 86
85 88
87 86.5
a
Weather data recorded in SANREM-CRSP automatic weather station at Bulogan, Lantapan.
inputs-outputs, market infrastructure, profitability, etc., in the Manupali watershed Poudel et al., 1998,
1999a. Seven preliminary transects within vegetable production zones were identified for diagnostic sur-
vey. These transects were discussed with and the interviewing schedule was reviewed by, leading veg-
etable farmers and political figures of the area. Eight barangays or villages Capitan Juan, Kaatuan, Alanib,
Sungco, Cawayan, Victory, Kibangay, and Basac were identified as survey sites representing vegetable
growing zones. The total number of vegetable grow- ers in each barangay was obtained from data recorded
by the Municipal Agricultural Office MAO, Lanta- pan, and the sample size was determined to represent
15 of that population. A composite random soil sample 0–15 cm was collected from the main parcel
of vegetable land and nearby uncultivated i.e. from under hedges or the field perimeters land for each of
the respondents surveyed. Each of these soil samples were analysed to determine the soil texture Day,
1965, soil pH in both 1 : 1 H
2
O, and 1 : 2 CaCl
2
so- lution, organic C Nelson and Sommers, 1982, and
exchangeable Ca extracted with NH
4
OAc at pH 7 and measured by atomic absorption spectrophotome-
try Blakemore et al., 1987. For the same parcel the representative natural slope and slope of the superim-
posed rows was measured with a Sunnto clinometer.
Results from the above survey were presented to the respondents in a 2-day farmers’ workshop organized in
April in the following year Poudel, 1995. Objectives of this workshop were to validate the survey results,
to identify indigenous technology systems that would be useful to minimize soil and nutrient losses from
vegetable fields, to determine future research activities on soil erosion control, and to identify farmer cooper-
ators for on-farm soil conservation research. Based on this highly interactive process, two issues on steep-
land vegetable production systems were identified: declining land productivity due to soil erosion, and
increasing incidence of diseases and pests. In order to minimize soil and nutrient losses and improve farm
productivity and income, a set of alternative technolo- gies to the traditional farmer up-and-down the slope
plantings, such as high-value contour hedgerows, contouring, and tree-vegetable intercropping were
identified as practices favoured by farmers for exper- imentation. Planting contour hedgerows across the
slope was identified as the most acceptable conserva- tion practice. However, most farmer participants did
not like the conventional hedgerow species because of the following reasons: 1 reduction in arable land
area, 2 shading of vegetable crops due to lateral spread over the field by e.g. Flemingia macrophylla
a leguminous tree, 3 requiring regular mainte- nance, 4 not providing immediate economic return,
and 5 unavailability of planting material. Napier grass Pennisetum purpureum K. Schum. was not
popular because its roots spread laterally. However, one farmer suggested making a ditch around the rows
of napier so that roots cannot spread out in the field. Farmers wanted to use high-value crops as hedgerows
in their vegetable fields. The following were the crops vegetable growers were interested to test as
hedgerows: asparagus, pineapple, pigeon peas, lemon grass, and tea. Results from tree-vegetable intercrops
are considered elsewhere Nissen et al., 1999.
2.3. Design and implementation of participatory field experiments
Field experiments to evaluate the effectiveness of high-value contour hedgerows and cropping sequences
in minimizing soil erosion were laid out at two levels of detail: researcher-managed, and farmer-managed.
D.D. Poudel et al. Agriculture, Ecosystems and Environment 79 2000 113–127 117
Fig. 1. Methodological framework for farmer participatory soil conservation research in the Manupali watershed, Mindanao, the Philippines.
Data were collected from both for seven cropping seasons.
2.3.1. Researcher-managed field experiment A researcher-managed field experiment was set
up in a site selected by the farmers and researchers. The researcher-managed site had 24 erosion-runoff
plots 19 m × 8 m each at on average 42 natu- ral slope. Each erosion-runoff plot was demarcated
with galvanized iron sheets at 22.5 cm above and 20 cm below the ground surface. Each erosion-runoff
plot had a leveled soil-collecting buffer at its base.
118 D.D. Poudel et al. Agriculture, Ecosystems and Environment 79 2000 113–127
These soil-collecting buffers were covered by tents to avoid mixing of eroded soil and direct rainfall
falling in the buffer zone, and each erosion-runoff plot had a pair of runoff-collecting barrels at the
bottom. The first barrel had 10 equal-sized holes, one linked to the next barrel with a connecting
hose to collect one-tenth of run-off water from the first barrel. Eroded soils were collected after every
rain event and weighed. This experiment tested 12 treatments four erosion conservation practices i.e.
contour hedgerows spaced between up-and-down cultivation of vegetables, contour planting of vegeta-
bles, strip cropping of vegetables and beans Phase- olus vulgaris L., and the farmers’ usual practice,
up-and-down cultivation of vegetables by three crops i.e. tomato, corn, and cabbage combinations in a
replicated randomized block design. There were three annual cropping sequences first crop planted in Jan-
uary: tomato-corn-cabbage, cabbage-corn-tomato, and corn-cabbage-tomato. A wide range of data were
collected from the researcher-managed site including, rainfall amount, soil and runoff losses, tillage prac-
tices, crop yields, crop cover, nutrient losses, and soil scouring. Composite soil samples 0–15 cm were col-
lected from original soil surface and at the end of the experiment for all erosion-runoff plots. Results from
this experiment are presented elsewhere Poudel et al., 1999b.
Table 2 Site characteristics and selected soil chemical properties of the original soil surface 0–15 cm on farmer-managed erosion-runoff plots in
the Manupali watershed, Mindanao, the Philippines Plot
Location Elevation
Natural Total-N
Organic C pH-H
2
O P
Ca Mg
K ID
masl
a
slope g kg
− 1
g kg
− 1
mg kg
− 1
cmol
c
kg
− 1
cmol
c
kg
− 1
cmol
c
kg
− 1
1 Sungco
1180 31
4 46
5.3 2.9
1.8 1.1
0.6 2
Mapawa 1305
23 4
53 5.1
1.8 1.6
1.3 0.5
3 Mapawa
1315 40
4 64
5.0 2.1
2.3 1.8
0.6 4
Mapawa 1345
33 4
78 5.0
2.1 1.2
1.1 0.4
5 Cawayan
1185 20
2 38
6.3 4.6
3.8 1.0
0.5 6
Cawayan 1200
16 2
43 4.8
2.1 2.7
0.9 0.3
7 Cawayan
1205 20
3 57
5.2 3.9
2.7 1.8
0.8 8
Victory 1210
37 5
73 5.9
8.1 7.4
2.0 0.9
9 Victory
1310 36
1 51
4.7 4.2
2.3 1.1
0.3 10
Kibangay 1280
44 3
46 4.7
4.9 0.9
1.1 0.5
11 Kibangay
1470 65
4 50
5.1 1.4
1.8 0.7
0.4 12
Basac 1000
62 4
43 5.0
3.2 1.7
2.1 0.9
a
Meters above sea level.
2.3.2. Farmer-managed field experiment There were 12 farmer-managed erosion-runoff
plots across the landscape of the Manupali watershed Table 2. As in the researcher-managed erosion-runoff
plots, the high-value species contour hedgerows in the farmer-managed erosion-runoff plots included
from top to bottom: asparagus, pineapple, pigeon peas, and lemon grass which replaced tea after the
first season. Slopes for the farmer-managed research plots ranged between 16 and 65 Table 2. As in
the researcher-managed erosion-runoff plots, each erosion-runoff plot was demarcated with galvanized
iron sheets set at 22.5 cm above and 20 cm below the ground surface. Each erosion-runoff plot had a
leveled soil collecting buffer at its base.
The number of contour hedgerows on farmer- managed erosion-runoff plots varied according to
their natural slope, as vegetable fields were placed into one of three categories: 25 slope, 25–40
slope, and 40 slope Poudel, 1995. The first category represented relatively gentle sloping areas
and were mostly plowed by draft animals. Vegetable fields under the second and the third categories were
cultivated without the use of draft animals. The ac- ceptable distances between contour hedgerows for
these three slope categories were: 7 m for 25 slope, 5 m for 25–40 slope, and 4 m for 40 slope
Poudel, 1995. Plots with 40 slope included all
D.D. Poudel et al. Agriculture, Ecosystems and Environment 79 2000 113–127 119
Table 3 Measured and simulated annual soil loss and simulated annual runoff on farmer-managed erosion-runoff plots in the Manupali watershed,
Mindanao, the Philippines
b
Plot ID Cropping sequence
Annual soil loss Simulated annual runoff mm
Measured Mg ha
− 1
Simulated Mg ha
− 1
1 Cabbage-fallow-tomato
a
16.5 11.0
56 2
Fallow-corn-cabbage 18.2
14.5 37
3 Cabbage-corn-potato
26.6 33.9
77 4
Fallow-cabbage-potato 34.0
45.3 81
5 Fallow-fallow-fallow
23.7 22.9
40 6
Cabbage-fallow-tomato 8.4
6.0 23
7 Sweet pepper-fallow-cabbage
13.4 11.8
33 8
Cabbage-potato-cauliflower 1.4
Na
b
Na
b
9 Fallow-fallow-potato
23.1 18.1
87 10
Potato-cauliflower-sweet pea 52.5
57.7 58
11 Fallow-fallow-potato
17.1 27.3
123 12
Fallow-fallow-potato 19.1
23.6 118
a
The first, second and the third crops represent January, May and September plantings, respectively.
b
Not available.
the four hedgerow species planted at 4 m intervals, those with 25–40 slope had asparagus, pineap-
ple and pigeon pea planted at 5 m intervals, and those with 25 slope had asparagus and pineapple
planted at 7 m intervals. Cropping sequences first crop planted in January–February in farmer-managed
erosion-runoff plots included: cabbage-fallow-tomato, fallow-corn-cabbage, cabbage-corn-potato, fallow-
cabbage-potato, fallow-fallow-fallow, sweet pepper Capsicum annuum L. var. annuum-fallow-cabbage,
cabbage-potato-cauliflower, fallow-fallow-potato, and potato-cauliflower-sweet pea Table 3.
Farmer-managed erosion-runoff plots were visited fortnightly by researchers to make sure that eroded
soils had been collected, weighed and recorded prop- erly in the data sheets provided to each farmer coopera-
tors. Eroded soils were collected after every rain event. Researchers made their visual observation on pest and
disease infestation, crop growth, erosion and weeds regularly. To ensure a better interaction between farm-
ers and researchers, visits were scheduled on those days when the farmers were available on their farms.
Data collection in farmer-managed research con- centrated mainly on soil erosion, tillage practices, crop
management, and inputs and output. For the first crop- ping season, eroded soils were collected, air-dried,
weighed, and recorded. However, farmers complained of the time and space needed for air-drying. There-
fore, they were provided a bucket to collect moist soil and record volumetrically. The wet soils were cali-
brated into dry weight based on researcher managed site. Air-dried weight was 40 of the wet weight. This
minimized farmers’ time and risk of loosing eroded soils while air drying. Except for plot number 10, 11
and 12 whose natural slope exceeded 40 Table 2, the first cultivation of these erosion-runoff plots was
done by a draft-animal drawn plow while all other cul- tivation and tillage practices were done by hand. All
cultivation and tillage practices were done by hand for plot number 10, 11 and 12.
Composite soil samples 0–15 cm depth were col- lected prior to planting the first crop July, 1995
and the end of the experiment August, 1997. Se- lected chemical properties were determined for soil
samples collected from both farmer-managed and researcher-managed erosion-runoff plots. Soil pH was
measured in 1 : 2 H
2
O. Organic C was determined by modified Walkley–Black method Nelson and Som-
mers, 1982, while total N was determined by the modified Kjeldahl method Black, 1965. Exchange-
able K, Ca, and Mg were extracted with NH
4
OAc at pH 7, and the cations in the leachate were measured
by atomic absorption spectrophotometry Blake- more et al., 1987. Available P was determined with
Bray-2 extraction Murphy and Riley, 1962. Selected soil chemical properties of the original soil surface
120 D.D. Poudel et al. Agriculture, Ecosystems and Environment 79 2000 113–127
0–15 cm on farmer-managed erosion-runoff plots are presented in Table 2.
2.4. EPIC modeling The EPIC model was calibrated using data for
the tomato-corn-cabbage cropping sequence from the researcher-managed erosion-runoff plots. The
tomato-corn-cabbage cropping sequence was selected because its annual soil loss 44.1 Mg ha
− 1
was similar to the average annual soil loss 45.4 Mg ha
− 1
for the three cropping sequences tested: tomato-corn-cabbage,
corn-cabbage-tomato and cabbage-tomato-corn in the researcher-managed site. The annual soil loss values
for corn-cabbage-tomato and cabbage-tomato-corn were 53.5 and 38.9 Mg ha
− 1
, respectively, suggesting that corn-cabbage-tomato sequence was the most ero-
sive of the three Poudel et al., 1999b. Higher soil loss in the corn-cabbage-tomato cropping sequence
is attributed to less canopy cover during the erosive months of August through October. EPIC model
calibration was also done for a fallow-fallow-fallow cropping sequence on farmer-managed plot 5. The
fallow crop best fit given by a substituted sorghum Sorghum bicolor L. Moench. hay crop to simulate
Imperata cylindrica L. Beauv. was established in October in 1995, and the calibration was undertaken
for fallow starting then.
The Green-Ampt infiltration equation available in the EPIC model was used to estimate runoff. Soil loss
was estimated using the small watershed version of the Modified Universal Soil Loss Equation. Weather files
were developed based on the 3 years’ weather data col- lected at the weather station in the watershed, which
was approximately 2 km from the researcher-managed experimental site. The weather file included daily
records on precipitation, temperature, solar radia- tion, and relative humidity. Since wind velocities
were lacking in this dataset, the Priestley–Taylor method that requires only radiation and temperature
was used to estimate the potential evapotranspiration. Soil information for each of the nine horizons from
the surface to 1.9 m depth was obtained from pro- file sampling and analyses Poudel and West, 1999.
Crop parameters used were from the USDA crop file provided in the model. The model was initialized as
close as possible with the measured annual soil loss values of tomato-corn-cabbage cropping sequence.
The model was validated with independent data sets from replicated cropping sequences: cabbage-tomato-
corn, corn-cabbage-tomato, fallow-fallow-potato, and cabbage-fallow-tomato from farmer-managed and
researcher-managed experiments, by comparing pre- dicted values to the measured values. The effective-
ness of the model for soil loss and runoff prediction under steepland vegetable systems was evaluated
using statistical measures including mean, standard deviations, and the root mean square error RMSE.
The RMSE values for each cropping sequence was calculated as follows:
RMSE = s
P
N i=
1
O
i
− S
i 2
N 1
where, O
i
are observed values and S
i
are simulated values, and N is the number of observations. This
method is commonly used to evaluate model perfor- mance Smith et al., 1996. The smaller the RMSE,
the closer the agreement between simulated and observed values.
Three-year simulation runs predicting annual soil loss and runoff with 15, 25, 35, 45, 55, and 65
slopes were made for each of the selected cropping se- quences: tomato-fallow-fallow, cabbage-fallow-fallow,
fallow-cabbage-fallow, tomato-corn-fallow, fallow- corn-cabbage, tomato-corn-cabbage, cabbage-tomato-
corn, cabbage-corn-tomato, tomato-cabbage-tomato, cabbage-tomato-cabbage and corn-cabbage-tomato.
These cropping sequences represent those commonly used for commercial vegetable production in the wa-
tershed Poudel, 1995; Poudel et al., 1998. Simulated plantings took place during the first week of January,
May and September, as is common in the region.
2.5. Farmer cooperators survey A brief survey primarily aimed at identifying
farmer cooperators’ perceptions, problems, and fu- ture suggestions in relation to FPR in soil erosion
management was conducted at the end of the on-farm field experiments. The survey included four main
open-ended questions: 1 what did you learn from the FPR? 2 what were the advantages of this research?
3 what were the major problems encountered dur- ing this research? and 4 what do you suggest for
D.D. Poudel et al. Agriculture, Ecosystems and Environment 79 2000 113–127 121
the future improvement of FPR on soil erosion? The survey information was analyzed and reported.
3. Results and discussion
