1. 8th INTERNATIONAL CONFERENCE OF EWRA “WATER RESOURCES MANAGEMENT IN AN INTERDISCIPLINARY AND CHANGING CONTEXT” PORTO, PORTUGAL, JUNE 2013
INTERACTION BETWEEN DROUGHT AND SOIL EROSION: THE CASE OF GUADIANA RIVER PILOT BASIN, PORTUGAL
E. Bakopoulos, P. Angelidis and V. Hrissanthou
Department of Civil Engineering
Democritus University of Thrace
67100 Xanthi, Greece

2. INTRODUCTION
Qualitative study of the interaction between drought and soil erosion
Portuguese part of Guadiana River basin
Separate quantification of drought and rainfall erosivity,
on which soil erosion depends
Drought indices: Rainfall deciles, Palmer Drought Severity Index (PDSI), Standardized Precipitation Index (SPI),
Reconnaissance Drought Index (RDI)

3. INTRODUCTION
Universal Soil Loss Equation (USLE): Estimate of the mean annual soil erosion
Product of five factors: rainfall erosivity factor, soil erodibility factor, topographic factor, crop management factor, erosion control practice factor
Assumption: The rainfall erosivity factor only varies significantly with time.
The variation of soil erosion with time is mainly due to the variation of the rainfall erosivity factor with time.
Rainfall erosivity factor: Product of two rainstorm characteristics:
kinetic energy (per unit area) x maximum 30-min intensity
Continuous record of rainfall intensity
Empirical regression equations between rainfall erosivity factor and rainfall amount

4. DROUGHT: STANDARDIZED PRECIPITATION INDEX (SPI)
Gamma probability density function:
x: amount of precipitation (x>0)
α: shape parameter (α>0)
β: scale parameter (β>0)
Γ(α): Gamma function
Cumulative probability function:
q: probability of zero precipitation
SPI: H(x) is transformed into the
standard normal distribution

5. MEAN ANNUAL RAINFALL EROSIVITY FACTOR
Modified Fournier Index (MFI)
Pi: mean monthly precipitation
of the month i (mm)
P: mean annual precipitation
(mm)
R: mean annual rainfall
erosivity factor
(MJ ha-1 mm h-1)
(for the Ebro basin, Spain)

6. MEAN ANNUAL RAINFALL EROSIVITY FACTOR
R: mean annual rainfall
erosivity factor
(MJ ha-1 mm h-1)
n: number of years
mj: number of erosive events
for a given year j
EI30: rainfall erosivity index for
a single event k
E: rainfall kinetic energy per
unit area (MJ ha-1)
I30: maximum rainfall intensity
in a 30 min period during the
event (mm h-1)
(Brown and Foster, 1987)

7. DAILY RAINFALL EROSIVITY INDEX
Rd: daily rainfall erosivity index
(MJ ha-1 mm h-1)
P: daily precipitation (mm)
α, β: parameters
m: current month
η: controls the amplitude of the
intraannual variation of β
ω: controls the phase
mmax: month registering the highest
average erosivity
(Yu and Rosewell, 1996)

8. GUADIANA RIVER BASIN Guadiana River flows through Spain and Portugal
Basin area: km2; km2 (83%) belong to Spain
11580 km2 (17%) belong to Portugal
River length in Spain: 550 km; river length in Portugal: 260 km
Natural border of both countries (transboundary river)
Guadiana River basin: typical semi-arid area of southern Europe

9. GEOGRAPHICAL SITE OF GUADIANA RIVER BASIN

10. GEOGRAPHICAL SITE OF 10 METEOROLOGICAL STATIONS (Daily rainfall data for the time period available)

11. ANNUAL RAINFALL DEPTH Meteorological station Amareleja

12. DROUGHT INDEX SPI 12 FOR THE METEOROLOGICAL STATION AMARELEJA

13. for all 10 meteorological stations
DROUGHT INDEX SPI 12
Positive values of SPI: designate wet periods
Negative values of SPI: designate dry periods
Negative slope gradient of the trend line: the region is prone to
drought
Negative slope gradient of the trend line:
for all 10 meteorological stations

14. RAINFALL EROSIVITY FACTOR
Determination of the parameters α and β in the equation for
Rd (daily rainfall erosivity index):
least squares method
Addition of the daily values Rd for a certain year to calculate
the annual value of the rainfall erosivity factor
The annual value of the rainfall erosivity factor can also be
calculated from MFI (Modified Fournier Index).
The sum of the squares of the deviations between the two annual values for all years considered, for a certain station, must be least.

15. ANNUAL RAINFALL EROSIVITY FACTOR
The annual rainfall erosivity factor was determined for older and newer years with the same almost annual rainfall depth.
Older years: time period
Newer years: time period

16. ANNUAL RAINFALL EROSIVITY FACTOR Meteorological station Amareleja

17. ANNUAL RAINFALL EROSIVITY FACTOR Meteorological station Amareleja
For 64.3% of 14 pairs of older and newer years, the annual rainfall erosivity factor of newer years is greater than the corresponding factor of older years.
The same trend resulted for the other 9 meteorological stations.

18. CONCLUSIONS The portuguese part of Guadiana River is prone to drought.
The rainfall erosivity factor (consequently the soil erosion also) displays an increasing trend in the newer years in comparison to the older years of the time period considered.
The rainfall intensity is decisive for the soil erosion, while the rainfall depth for the drought.
If the denudation of the soil because of the drought were taken into account, then the soil erosion would be more severe.