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Hazards: the impact of erosion on the agricultural environment

The combination of a mountainous landscape and heavy population pressure on the land is the cause of the chronic erosion found in the Andes. Along the whole length of the Andean depression, the frontiers of colonization have been pushed further and further back in the space of a few years, with a proportionate increase in the forms of erosion and in the amount of abandoned land.

EXTENT OF AGRICULTURAL EROSION

According to the results of a joint survey by the Ecuadorian Ministry of Agriculture and Animal Husbandry and ORSTOM on the main processes of erosion in Ecuador (Almeida, De Noni et al. 1984, De Noni and Viennot 1987), 50% of the country's area (70% of it in the Andes, and 30 % in the coastal and Amazonian regions) is affected by processes of degradation. In the Andes, the most degraded region, there are two distinct zones:

• the intra-Andean basin (1500 to 3000 m) where very little arable soil is left; in the northern and central sections of the basin there is a formation of hardened volcanic ash, locally called cangahua, remarkable for its extent and its depth, and sterile for agricultural use; this formation appears when soil and volcanic ash have been scoured by erosion;

• the highlands and outer slopes of the two ranges (3200 to 4000 m) where active erosion develops as the agricultural frontier advances - as it has continuously over the last twenty years. Although soil cover is still present throughout these regions, it is showing alarming signs of degradation in some places.

FIGURE 83
Soil erosion and conservation in Ecuador (DNA-ORSTOM project)

Figure

Erosion in Ecuador

Main units in terms of landforms and soil

MAIN UNITS IN TERMS OF LANDFORMS AND SOIL

Landforms	Caldera, neck	Gullied slopes, cones 1 3 4	Pediment-terrace 5	Horst, graben 6 7	Extinct volcano 8
Soils	cryandept orthent	dystrandept hydrandept	arguistoll hapludoll	haplustoll/cangahua	cangahua durustoll
OM %	9 to 12	6 to 15	5	3 to 4	< 1
pH	5.5	5 to 6	> 7	> 7	7 to 8.5
CEC meq/100 g	2 to 10	15 to 40	20 to 80	20 to 50	20 to 30
T/S %	10 to 80	2 to 50	80 to 100	100	100
Granulometry	SL	L	AL	AL	LSA
Remarks	coarse elements	H% 50 to 250%	smectite		hardened CO3Ca

TABLE 44
Soil loss on 50 m² plots (1981-84 and 1982)

Year	ALANGASI		ILALO
	Mollisols and cangahua		Moilisols and cangahua	Cangahua
	Various treatments (maize, pasture)	Degraded pasture	Maize	Fallow
	t/ha	t/ha	t/ha	t/ha
1981-84	62	314	631	71
1982	58	204	421	58

PREDOMINANCE OF WATER EROSION

During the nine-month Sierra cropping cycle from September to May, water erosion is very active (wind erosion is not considered here, since it is more localized and has little effect on crops), mainly in the following forms (De Noni and Trujillo 1989):

• diffuse and concentrated runoff is the most widespread form, whatever the geological origin of the soil - pyroclastic formations in the northern and a large part of the central Sierra, and volcanic-sedimentary material in Loja province in the south; land affected by this process has shallow soil with truncated horizons, and is scored by erosion in U or V shapes according to how cohesive and granular the soil is; these linear forms quickly develop into badlands;

• runoff combined with small mass movements (15 to 20% slopes), a process seen in soils with a discontinuous texture, where a clayey soil of volcanic origin, rich in montmorillonite, lies over a hard cangahua-type formation; the processes work together, shaping the soil surface into eroded precipices as high as 3 or 5 metres; this happens in the northern and central parts of the Sierra (Carchi, Pichincha and Chimborazo provinces);

• mass movement is confined to the Cumbe area, south of the Cuenca basin, where the whole landscape has a moutonné appearance; erosion takes the form of creep, with humps and hollows developing in hilly landscapes with clayey non-volcanic soil.

HIGHLY ACTIVE RUNOFF PROCESSES

Other studies by the DNA-ORSTOM project on 50 m: cultivated runoff plots (10 × 5 m) also show the extent of runoff caused by human activity. Table 44 groups together soil loss for the period 1981-1984, as recorded on two plots in the Quito basin - at Alangasi on a 28% slope and at Ilalo on a 33 % slope (De Noni, Nouvelot and Trujillo 1984 and 1986). The year 1982, during which most of the erosion occurred, is shown in a separate column.

TABLE 45
Annual soil loss and rainfall on 100 m² plots

		Year 86-87			Year 87-88
		Total annual rainfall mm	Control plot with crop	Bare tilled fallow	Total annual rainfall mm	Control plot with crop	Bare tilled fallow
TUMBACO	Erosion t/ha/yr	478	3.02	12.9	457	42.18	82.82
	Runoff coeff. %		3.7	6.6		4.4	15.5
CANGAHUA	Erosion t/ha/yr	366	3.8	56	308	6.89	83.6
	Runoff coeff. %		1.6	5.9		1.9	11.1
MOJANDA	Erosion t/ha/yr	588	1.15	5.9	547	0.52	96.94
	Runoff coeff. %		0.9	2		1	8.4
RIOBAMBA	Erosion t/ha/yr	537	1.44	56.9	532	52.2	198.7
	Runoff coeff. %		0.9	23.3		15.5	21.3

TABLE 46
Development of agricultural production in the Sierra 1970-85 (in tonnes)

CROPS	1970	1975	1980	1985
Barley	79087	62801	24350	26723
Maize	167990	90247	45266	35421
Wheat	81000	64 647	31113	18464
Potato	541794	499371	323222	423186

Since 1986 the project has expanded its scope, establishing larger runoff plots (20 × 5 m = 100 m²) located throughout an area stretching 800 km along the Sierra, from Pichincha province in the north, to Loja province in the south. There are two types of plot: a bare, tilled plot, in accordance with Wischmeier's prescription (Wischmeier and Smith 1978, Roose 1968) and a control plot using local crops and practices. The bare Wischmeier plot is not only a fundamental scientific reference plot, but in the present case also reflects the real features of the farm year - fallow poor in leafy vegetation and soil bare at the time of sowing barley. For the period 1986-88, the results are given in Table 45.

The following are the main lessons to be drawn from the results:

• Events of spectacular erosion on cultivated soil: for example, on the 50 m² plots monocropped with maize, erosion exceeded 600 t/ha.

• Irregular erosion from one year to another: at Alangasi end Ilalo soil losses in 1981, 1983 and 1984 were low compared with 1982, when most of the erosion occurred. The -results also vary considerably from one year to another on the individual station; for example, in 1987-88 16 times more soil loss than in 1986-87 was recorded at Mojanda, and 7 times more at Tumbaco on the Wischmeier plots, and 32 times more at Tumbaco on the control plot.

• The absence of a regular erosion season during the year (most of the erosion is the result of the five most erosive rainstorms, out of a total of some forty erosive rainstorms per year per station): the amounts of rain in the individual rainfalls are not the only explanation of soil losses. In calculating correlation coefficients for the years 1986-87 and 1987-88, it appears that the best correlations with the weight of eroded soil are recorded with IM15 or IM30, although the maximum intensities in the Sierra are only low to average: 15 to 45 mm/in. The "R" values on the erosivity index are therefore only moderate, rarely exceeding 100: 60 at Cangahua, 90 at Tumbaco, and 100 to 110 at Mojanda and Riobamba. However, all these figures are also subject to annual variations, and can be considerably higher and extremely erosive. On the Ilalo plot (50 m²), erosion was over 400 t/ha/yr in 1982; an IM15 of 90 mm/in was responsible for 270 t/ha/day of lost soil, and another of 70 mm/in for 120 t/ha/day.

ABSENCE OF CONSERVATION METHODS AND FOOD DEFICIT

The history of land use clearly brings out two distinct types of farmer: rich landowners on the haciendas, and marginalized peasant farmers on the minifundios. The latter are forced by sheer necessity to produce more in order to survive: driven into a marginal environment, they have been obliged to push the environment beyond its limits, with the result that all along the Sierra today there is a striking absence of erosion control practices suited to the environment (De Noni, Viennot and Trujillo 1986). For example, on the densely cultivated highlands of Chimborazo and Cotopaxi provinces, there are some perfunctory soil conservation structures consisting of hedges and small ditches. The latter are very shallow (20 to 40 cm) and steeply sloping (20 to 25%) in comparison with a conventional ditch, so that they cannot channel excessive water, and rapidly become gullies. Similarly, hedges, usually composed of sigses (Gybernium), are placed randomly in relation to the main slope. Moreover, these structures, set on the edges of plots, are only rarely combined with contour tilling.

There is also systematic destruction or abandonment of early agricultural structures inherited from pre-Columbian societies. These remains (Gondard 1983), mostly bench terraces, are made with risers built of stone or blocks of hardened ash. At Pimampiro in Imbabura province, the risers are deliberately knocked down to make room for large plots mechanically tilled in the direction of the slope. Similarly, near Zhud in Cañar province in a recently settled area with medium-sized holdings, wide concave terraces dating from the Cañari civilization are appearing from beneath the shrubby vegetation (chaparral) during clearing. At Punin and Flores, as well as Colta and Chunchi, all in Chimborazo province, at an altitude between 3200 and 3600 m, and on steep slopes (40 to 60%), there are real terraces separated by risers several metres high. Here again, the intermediate risers have been abandoned or destroyed, leaving only those that lie along property boundaries, so that they now border excessively wide and sloping "pseudo-terraces", which are totally unsuited to local conditions.

Certainly in order to survive, but also in the hope of entering the market economy, the small farmers have oriented the minifundios toward the crops that provide the basic local foodstuffs cereals (maize, wheat and barley) and tubers (potatoes). However, natural limitations have prevented the development of profitable farming, so that the situation of the minifundios is now very precarious: in some regions self-subsistence is barely assured, while surplus production is rare and depends on a year of exceptional yields. Ministry of Agriculture data on levels of farm production over the past 15 years clearly illustrate this crisis situation (Table 46).

TABLE 47
Conservation methods tested on improved 1000 m² plots

Stations	Slope %	Crops		Conservation methods	Soil losses t/ha/yr		Annual runoff coefficient %
		86-87	87-88		86-87	87-88	88-87	87-88
TUMBACO	20	maize	maize	grass strips with three forage species	1.08	0.42	1.5	0.8
CANGAHUA	20	maize	maize	cangahua walls	0.45	0.33	0.1	0.5
MOJANDA	40	barley/potato/beans		sod walls and large quinoa covered ridges	0.38	0.19	0.3	0.1
RIOBAMBA	20	potato/beans/barley		grass strips with mixed cropping (barley, beans)	0.43	7.6	0.3	6.2

It can be seen that there was a huge and generalized fall in yields of all these crops between 1970 and 1980, particularly for maize and wheat, which form the traditional staple diet of the rural population.

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