Mr. Rafael Giménez, Public University of Navarre, Spain. Global Symposium on Soil Erosion (GSER19), 15 - 17 May 2019 at FAO HQ.

1. Long-term monitoring of sediment yield in typical
agricultural watersheds of Navarre (Spain)
1
Giménez, R.1, Casalí, J.1, Merchán, D.1, Campo, M.A.1, Goñi, M.1,
Del Valle de Lersundi, J.2
1Department of Engineering, IS-FOOD Institute, Public University of Navarre,
Spain
2Department of Rural Development, Environment and Local Administration,
Government of Navarre

2. 2
Soil erosion in agricultural lands is an important problem in Navarre (Spain).
Local government established a network of experimental watersheds for assessing
the impact of agricultural activity on soil and water resources.
http://cuencasagrarias.navarra.es/index.cfm
INTRODUCTION

3. 3
These watersheds are representative of wide areas of Navarre and Spain
regarding morphology, soils, climate, land use and management.
http://cuencasagrarias.navarra.es/index.cfm

4. 4
Experimental watersheds provide the
possibility of studying soil erosion from
a systemic point of view relating the
different factors involved such as types
of land use.
Why studies at watershed scale?
Poesen and Hook (1997)
Types of erosion features in agricultural lands

5. 5
Rainfall, runoff and sediment yield continuously recorded at the outlet of 4
agricultural watersheds with contrasting land use during 2007-2016 are evaluated.
OBJECTIVE
To analyse the long-term behaviour of agricultural experimental watersheds in
terms of sediment yield and discharge.

6. 6
Landazuria
PAMPLONA
La Tejería
Oskotz
Latxaga
Navarre’s experimental watersheds
MATERIAL AND METHODS
~200 ha
~200 ha
~1600 ha
Winter cereals
Winter cereals
Forest-pasture
Irrigated crops
Rainfed cereals
~500 ha

7. Experimental watersheds: measurement devices

8. 8
Automatic weather station
(i) air temperature, (ii) rainfall, (iii) air moisture, (iv) wind speed/direction, (v)
soil temperature, (vi) solar radiation
Monitoring on a 10 min basis:

9. 9
Hydrological station
In Oskotz there are 2 hydrological stations:
Oskotz principal (Op) monitors the whole watershed (forest and pasture)
Oskotz woodland (Ow) is a subwatershed almost fully covered with forest
Ow

10. 10
Crump weir
cavity for water and sediment sampling
Discharge and sediments measurements
Limnimetric scale
Hydrological station

11. 11
Sediment and solutes automatic sampler
ISCO 3700 sampler
4 samples a day
Flood sampler
Hydrological station

12. Turbidimeter
Hydrological station

13. turbidity recorded every 10 min
Estimation of sediment concentration from turbidity

14. Tiempo(min)
0 5000 10000 15000
Q(m3/s)
0,00
0,02
0,04
0,06
0,08
0,10
Sedimentos(g/l)
0
1
2
3
4
5
P(mm)
0,0
0,5
1,0
1,5
2,0
Caudal(m
3
/s)
Sedcalculado(g/l)
Sedmedido(g/l)
Estimation of sediment concentration from turbiditySediments,g/l
Turbidity, NTU
Discharge (m3/s)
Sed. measured (g/ l)
Sed. estimated (g/l)
Sediments, g/l
Turbidez (NTU)
0 100 200 300 400 500
Sedimentos(g/l)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
y= 0.0068x - 0.0398
R2
= 0.9647
Sediments, g/l

15. 15
Sediment yield. Landazuria, presented the lowest values followed by Oskotz
(Principal and Woodland), Latxaga and La Tejería.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
La Tejería Latxaga Oskotz P Oskotz W Landazuria
Averagesedimentyield(Mg/ha)
(87) (123) (76) (83) (172)
(CV%)
RESULTS

16. 16
Monthly average rainfall and runoff at La Tejería watershed (2007-2016). Vertical bars are
standard deviation. Similar pattern was observed in the remaining watersheds.
In all the watersheds rainfall and runoff showed a seasonal pattern: winter, the wettest
period; summer, the driest. High inter-annual variability especially in winter.
General behaviour

17. 17
Winter rainfalls have low erosivity while the scarce summer rainfalls have high erosivity
Average fortnightly and accumulated average fortnightly values
of rainfall erosivity (EI30) at La Tejería watershed
General behaviour

18. 18
Winter. Low erosive rainfalls. But soils are usually almost saturated, and the
vegetation cover is scarce: large runoff rates flowing over unprotected soils.
Summer. Higher rainfalls erosivity. But more protected and drier soils (i.e., more
infiltration capacity).
Vegetation cover varies along the year especially in the grain-producing watersheds
Sediment yield mainly occurred during winter and beginning of springtime.
General behaviour
50 cm
Soil macropores/cracks
under dry conditions

19. Circular shape, flatter topography,
steeper, non vegetated stream channels
promote larger peak discharges and then
sediment yield.
650 m
650 m
490 m
LaxtagaLa Tejería
Complex topography, gentler
slopes, vegetated stream channels
provoke sedimentation of eroded
particles before reaching the
watershed outlet.
La Tejería presented 3 times more sediment yield than Latxaga. This may be explained
by different morphological characteristics and vegetation cover in stream channels.
Particular behaviour
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
La Tejería Latxaga Oskotz P Oskotz W Landazuria

20. 20
In La Tejería erosion features are eventually observed in some subwatersheds
mass movements bank erosion
Particular behaviour

21. 21
In Oskotz sediment yield is relatively low despite the dominant steep hillslopes. This,
because the almost permanent coverage of soils by pastures and forest.
BUT
Particular behaviour
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
La Tejería Latxaga Oskotz P Oskotz W Landazuria

22. 22
Oskotz Forestal
02 03 04 05 06 07 08 09
Concentraciónsedimentos(gl
-1
)
0
10
20
30
40
Evacuacióndiariasedimentos
(kgha
-1
día
-1
)
0
200
400
600
800
1000
1200
1400
Sedimentosevacuados
acumulados(kgha
-1
)
0
250
500
750
1000
1250
1500
1750
Accumulatedsedimentyield(kg/ha)
Sedimentconcentration(gr/l)
Dailysedimentyield(kg/haday)
Oskotz woodland
years
Firewood
extraction, 10 ha
Oskotz is prone to eventually suffer from high erosion rates due to forest clearance
(e.g., Oskotz woodland, 2003)
Particular behaviour

23. 23
In Landazuria sediment yields is also relatively low probably due to a better vegetation cover
along the year promoted by irrigation and a more o less flat relief.
Particular behaviour
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
La Tejería Latxaga Oskotz P Oskotz W Landazuria

24. 24
Sediment yield is more related to current soil conditions than to the erosive
capacity of the rains.
An important inter-annual variability of sediment is observed, typical of our
climate.
However, erosion rate is highly variable depending on a complex (inter)action
of a number of natural and man-induced factors such as differences in
morphology, topography, vegetation cover of the soils and/or vegetation cover
of the stream channel of the watersheds.
CONCLUSIONS

25. 25
The pastures and forest afford a greater protection against soil erosion than the
crops (cereals).
However, logging practices can eventually trigger important increases in sediment
yield since they leave part of the soil’s surface bare and compacted.
For a correct assessment of soil erosion at watersheds scale in Mediterranean
condition an extensive (decades) and continuous dataset is needed.
CONCLUSIONS
This dataset is of great utility to implement sound management practices and to
evaluate hydrological-erosion models.