Effect of reforestation on annualwater yield in a large watershed in northeast China

Yuefeng Yao•Tijiu Cai•Cunyong Ju•Chengxin He

Effect of reforestation on annualwater yield in a large watershed in northeast China

Yuefeng Yao1•Tijiu Cai2•Cunyong Ju2•Chengxin He1

A simplified water balance model in conjunction with an evapotranspiration(ET)modeland cumulative forest cover data were used to quantify the changes in annual water yield in response to reforestation in a large watershed,northeast China.Cumulative forest cover increased by 22%,leading to a significant decrease in estimated annual water yield.Reforestation increased ET (P=0.0144),resulting in a remarkable decrease (P=0.0001)in estimated annual water yield according to the water balance model.Reforestation increased ET by 33 mm and decreased annual water yield by 38 mm per decade.The effect of reforestation on annual water yield can be quantified using a simplified waterbalance modelin a large watershed,although our reforestation area was small(about20%)in relation to the total watershed area.

Annual water yield·Cumulative forest cover·Evapotranspiration(ET)·Reforestation· Water balance model

Introduction

The effect of timber harvest on stream flow was evaluated using a paired watershed method at Wagon Wheel Gap in Colorado in 1909(Bates and Henry 1928).Since then,the effects of forest disturbances on hydrological regimes have received increased attention(Bosch and Hewlett 1982;Andre´assian 2004;Brown et al.2013).Studies of the effects of forest disturbances on stream flow within watersheds were reported by Zahabiyoun(1999)using catchment experiments and models.In small catchment experiments,increased stream flow followed deforestation and decreased stream flow followed reforestation(Brown et al.2005).These changes in stream flow were detected after at least 20%of the catchment area had changed. However,changes in hydrological regimes are not expected to be similar in larger scale watershed studies (Costa et al.2003;Zhao et al.2009;Buttle 2011).For example,when logging ranged from 5 to 25%ofthe total area of five different large watersheds(＞1000 ha)in Ontario,Canada,there were no observable changes in yearly runoff because the timber harvested areas accounted for a small proportion of the total watershed area(Buttle and Metcalfe 2000).Wilk et al.(2001) reported on a study ofthe 12,100 ha Nam Pong watershed in northeast Thailand where,despite a 53%reduction in forest cover,there were no detectable changes in stream flow.Quantifying the effects of forestdisturbances on hydrological regimes remains a challenge(Stonestrom et al.2009).

Hydrological models have widely been used to analyze and predict the behavior of stream flow in response to land use and cover changes(Croke etal.2004;Siriwardena etal. 2006).However,when extrapolated to ungauged watersheds where no measured data exist,using these models is challenging(Breuer et al.2009).The mostdirect effectcaused by climate and land use and cover changes on hydrological regimes is on the magnitude and distribution of evapotranspiration(ET),and,consequently,change in stream flow and water quality(Dow and DeWalle 2000).The water-energy balance equation has been used to calculate ET,then to evaluate the changes in stream flow(Zhang etal.2001;Yang et al.2009;Renner and Bernhofer 2012).Milly(1994) developed a mathematical framework to calculate annual mean ET and assesscatchment-scale vegetation changes,but this requires complex numericalsolutions.The ET modelof Zhang et al.(2001)includes a fractional forest cover parameter(f)and two plant-available water coefficients for forest(w1)and non-forest areas(w2).This model fit data from 250 catchments worldwide.In addition,Sun et al. (2006)used a simplified hydrologicalmodelin combination with the Zhang etal.(2001)ET modelto estimate the effect of forestation on annualwater yield.Given thatthe amount of precipitation was constant and the change in soil water storage during pre-and post-forestation periods was similar, they concluded that forestation has the potential to reduce annualwater yield by 50–300 mm per year.

China has experienced unprecedented changes in forestation,in particular after launching several large-scale reforestation programs in the late 1990s(Wei et al.2008). These changes are expected to continue into the twentieth century,especially in northeast China where the availability of water resources has become one of the most important factors influencing agricultural development (Liang et al.2011).Forestation programs account for the largest fraction of land use changes and,in some cases, have the primary impact on hydrological regimes(Calder 2000).Forestation impacts on hydrological regimes have significant implications for forestation programs implemented to sequester carbon and/or manage water resources. Therefore,the objective of this study was to apply a simplified water balance modelin conjunction with the Zhang etal.(2001)ET modeland cumulative forestcover data to quantify the effects of reforestation on annual water yield from 1973 to 2006 in a large watershed,northeast China.

Materials and methods

Study area

The Xinancha watershed is located in the Xiaoxing’an mountain range south of Yichun City in Heilongjiang Province,northeast China(Fig.1).Itcovers a totalarea of approximately 2582 km2.It has a monsoon and humid continental climate and receives mean annual precipitation of 750 mm of which the monsoon season(June–August) accounts for more than 70%.Mean annual air temperature is 1.21°C.The topography ofthe watershed is diverse.Itis characterized by low mountains,hills,valleys and plains. The main soil types include dark brown earths,brown coniferous forest soils,meadow soil,and calcic chernozems.The soils in most of the forested areas are dark brown and brown coniferous forest soils.They are loamy, with rich soil organic content ranging from 68 to 118 g kg-1(Wang 2013).The soil capillary water storage capacity ranges from 64 to 211%in the A horizon,and 33 to 134%in the B horizon(Zhao 2010),respectively.The vegetation is boreal forest,particularly northern coniferous-deciduous mixed forest,predominantly consisting of Korean pine,spruce,Chinese larch and oak.

Data

Annual forest cover data from 1973 to 2006 were obtained from the forest resource inventory database of the State Forestry Administration in Yichun City.The percentage of forestcoverwascalculated asthe sum ofallthe forested land areas divided by the total land area of the Xinancha watershed.China has implemented reforestation programs ofvarioussizessince the 1970s to meetthe demand fortimberand to ameliorate environmental damage.This is especially important in northeast China because of the extensive deforestation thatoccurred from the 1950s to the 1970s(Wei etal.2008).Cumulative forestcoverin the Xinancha watershed expanded gradually between 1973 and 1979,was followed by minor infilling between 1979 and 1991,and increased gradually again between 1991 and 2006(Fig.2).

Annualwateryield data for 1973 to 2006 were obtained from the Yichun Hydrological Bureau and the Heilongjiang Hydrological Yearbook.Climatic data,such as mean,maximum and minimum temperature,and precipitation were collected from four climate stations within the Xinancha watershed,namely Xiaobai(46°58′N,128°42′E), Liangshui(47°11′N,128°53′E),Dailing(47°02′N, 129°01′E)and Nancha(47°08′N,129°15′E).The value for each climate factor from 1973 to 2006 was calculated by averaging the total of the four climate stations using the Thiessen polygon delineation method.

Methods

Annual average water yield for a watershed can be described as follows(Sun et al.2006):

Fig.1 Xinancha watershed located south of Yichun City in northeast China

Fig.2 Cumulative forest cover(%)over time for the Xinancha watershed

where,Q,P,ET andΔS are annual water yield,precipitation,evapotranspiration and change in water storage, respectively.

Equation 1 can be expressed as:

Assuming no change in soil water storage during the study period,the change in Q due to forestation is the change in the difference between precipitation and ET. Q±ΔS is considered as the estimated annual water yield (Qe).Therefore,Eq.2 can be described as:

ET was calculated according to the Zhang et al.(2001) ET model as:

where,w is the plant-available water coefficient,represents the relative differences in water use for transpiration,and was reported as 2.0 for forests(Zhang et al.2001).PET is potential evapotranspiration.Annual PET was calculated using airtemperature and day length using the Hamon PET method,which is a simpler but comparable method that requires fewer climatic parameters than other PET methods (Farley et al.2005).

ET for a forested watershed with a certain percentage of forest cover(f)equals to ET by f.

Thus,the effects of forestation on annualwater yield can be approximated mathematically as:

Results

Trend analysis of precipitation and annual water yield

By linear regression analysis,neither annual precipitation nor annual water yield changed significantly during the entire study period from 1973 to 2006(Fig.3)(P＞0.1), although both had slightly decreasing trends.Average annual precipitation was 631 mm with a maximum of 936 mm in 1994 and a minimum of 360 mm in 2001. Average annualwater yield was 289 mm,with a maximum of 608 mm in 1994 and a minimum of 100 mm in 2001. The correlation coefficient of annual precipitation and water yield was 0.8787(P＜0.0001),indicating that annual water yield was sensitive to precipitation changes.

Effect of forest cover on annual water yield using the water balance model

Because the measured annualwater yield failed to show an increasing or decreasing trend during the study period,we used estimated annual water yield(Qe),calculated as the difference between precipitation and ET of forested areas, to evaluate the effects of reforestation on annual water yield.Although the increase in forested area accounted for about 22%of the entire watershed area over the study period,ET was affected,resulting in changes to annual water yield.There was significant positive correlation between ET and cumulative forest cover(P＜0.05). Reforestation resulted in a significant increase in ET (P=0.0144)(Fig.4),which led to a significantdownward trend(P=0.0001)in Qe(Fig.5).This indicates that reforestation decreased annual water yield by increasing ET.Reforestation increased ET by approximately 33 mm per decade and decreased the Qeapproximately 38 mm per decade over the course of the study.

Fig.3 Annual precipitation and water yield in the Xinancha watershed between 1973 and 2006.Trends were not significant (P＞0.81)for both variables

The ET ratio(ET/precipitation)and Qeratio(annual water yield/precipitation)were used to remove the interannual variability effect of precipitation(Fig.6).The ET ratio displayed a significantly increasing trend (P＜0.0001).By contrast,the Qeratio had a significantly decreasing trend(P＜0.0001),suggesting that reforestation decreased the annual water yield by increasing ET.

Discussion

China has the largest area of forest plantations,approximately 45 million ha or 1/4 of the world total(FAO 2001). Massive forestation activities are expected in China in future decades(Sun et al.2006).Much progress has been made in understanding the relationships between forest cover and hydrology during the past decades(Liu et al. 1996;Wei et al.2008).Xinancha watershed,a large watershed in the temperate zone of northeast China, experiencing no significant change in annual precipitation and an expansion of forest coverage from 1973 to 2006, provided a usefulcase for evaluating the relations between reforestation and hydrology.Annual water yield was estimated as the difference between measured precipitation and actualET.We assumed no change in soilwaterstorage during the study period as Sun et al.(2008)and Li et al. (2014)did.Our result agrees with the general conclusions reached for small watersheds that there were detectable changes in annual water yield when at least 20%of the forest cover was re-established(Bosch and Hewlett 1982; Moore and Scott 2005).

Fig.4 Upward trend of forest area and ET between 1973 and 2006

Fig.5 Downward trend of estimated annual water yield(Qe)

Fig.6 Upward trend of ET ratio,and downward trend of Qeratio between 1973 and 2006

The changes in stream flow as a result of conversion to different types of vegetation are mainly due to changes in ET(Brown et al.2005;Turner 1991).Compared with grasslands and other short crops,forests have greater leaf areas and root systems that access deeper water resources so forests usually have higherevaporative waterlosses and, thus,reduced stream flow(Zhang etal.2001;Jackson etal. 2005;Vanclay 2009).Therefore,an increase in the proportion of forested land area in a watershed could cause a reduction in annual water yield.On a global scale,reforestation leads to reductions in annual stream flow and low flows(Farley et al.2005).However,in many countries, forest protection and reforestation are promoted within watershed development programs(Calder 2004).China continues to implementforestprotection programs,such as the Natural Forest Protection Program(NFPP),the Grain for Green Project(GGP),and other shelterbelt development programs(Wang et al.2011a).It is thought that forests can sequester more carbon than other land use types,ameliorate environmental degradation,and lead to sustainable social and economic development.However, reforestation can also potentially reduce stream flow.Our study indicates that reforestation increased ET,and,thus, decreased annual water yield.This is consistent with other findings that increased potential evapotranspiration due to reforestation and reduction of precipitation due to climate change will further aggravate the water yield reduction in northern China(Liu and Fu 1996).The effectof foreston hydrology can be just as important as that of climate change(Weiand Zhang 2010;Zhang etal.2012;Liu etal. 2014).Expansion of reforestation might increase evapotranspiration and consequently cause reduction of stream flow(Liu et al.2014).

The effect of reforestation on annual water yield was evaluated by comparing the changes in annual ET attributed to forest cover in this study.We considered the estimated annual water yield as the difference between precipitation and ET offorested land area in the watershed. Our assumption of no change in water storage during the study period might affect the accuracy of the estimated annual water yield in the water balance model,especially for large scale watersheds thatdemonstrate greatvariability in precipitation and available water capacity.Moreover, different types of forests have different ET.The w parameter,setat2.0 for forests,mightmisrepresentthe ET when using the Zhang et al.(2001)ET model.Therefore,as Wang et al.(2011b)mentioned,there is uncertainty in assessments of the impact of forest disturbances on stream flow.

AcknowledgmentsThis work was financially supported by the State Forestry Administration of China(201404201),the Guangxi Natural Science Foundation of China(2012GXNSFBA053140),the Fundamental Research Funds of Guangxi Academy of Sciences (13YJ22ZWS22)and the Guangxi Institute of Botany(12001).This paper also was supported by CFERN&GENE Award Funds on Ecological Paper.The authors thank the Ministry of Forests and the Hydrological Bureau of Yichun City for providing assistance during our field investigation and data collection.We also appreciate the constructive comments and suggestions from two anonymous reviewers to improve the quality of our manuscript,and thank Stephen Maciejewski for language correction.

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1 July 2014/Accepted:10 November 2014/Published online:18 July 2015

©Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

Project funding:This work was financially supported by the State Forestry Administration of China(201404201),the Guangxi Natural Science Foundation of China(2012GXNSFBA053140),the Fundamental Research Funds of Guangxi Academy of Sciences (13YJ22ZWS22),the Guangxi Institute of Botany(12001),and CFERN&GENE Award Funds on Ecological Paper.

The online version is available at http://www.springerlink.com

Corresponding editor:Yu Lei

✉Tijiu Cai yuefeng.yao@yahoo.com

1Guangxi Key Laboratory of Plant Conservation and Restoration Ecology in Karst Terrain,Guangxi Institute of Botany,Guilin 541006,China

2School of Forestry,Northeast Forestry University, Harbin 150040,China