8th INTERNATIONAL CONFERENCE OF EWRA “WATER RESOURCES MANAGEMENT IN AN INTERDISCIPLINARY AND CHANGING CONTEXT” PORTO, PORTUGAL, 26-29 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

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)

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

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

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)

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)

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)

GUADIANA RIVER BASIN Guadiana River flows through Spain and Portugal Basin area: 66800 km2; 55200 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

GEOGRAPHICAL SITE OF GUADIANA RIVER BASIN

GEOGRAPHICAL SITE OF 10 METEOROLOGICAL STATIONS (Daily rainfall data for the time period 1932-2010 available)

ANNUAL RAINFALL DEPTH Meteorological station Amareleja

DROUGHT INDEX SPI 12 FOR THE METEOROLOGICAL STATION AMARELEJA

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

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.

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 1930-1980 Newer years: time period 1980-2010

ANNUAL RAINFALL EROSIVITY FACTOR Meteorological station Amareleja

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.

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.